I know there's a lot of beef out there with AP Gragas' name on it, but I can assure you that if you get good with him, he is completely viable early, mid, and late game (Maybe falling off a bit around the 55 minute mark, but who plays games that long anymore)
Remember, it's not necessarily the build that matters for AP Gragas, it's how you use his abilities to your advantage. Gragas has ways of setting up all 3 of his damage dealing abilities into one giant, floppy DPS burst, so long as you know when to use them and how.
Summoner Spells:
Clairvoyance, Exhaust
Exhaust: Once you blind any DPS Characters you are guaranteed to land most of your abilities on them, and either they will run away from the burst, or they will die from it. Your fists weren't meant for fighting, they were meant for rolling. Remember that.
Clairvoyance: Manna from heaven! This ability will score you plenty of ganks mid and early game, and it's usefulness for the whole team late game is obvious. Great for keeping people out of bushes and getting easy barrel rolls too.
Item Build:
Focusing on Magic Penetration early is a good investment for later, and Rod of Ages will cover you on the HP, Mana, and Ability Power fronts well past the time you buy Void Staff. Only real problem with this build I've found is a lack of Mana Regeneration especially early game. Haunting Guise sometimes isn't quite enough.
1) Rejuv. Bead+Faerie Charm ---> Haunting Guise
2) Boots
3) Rod of Ages
4) Boots ---> Sorcerer's Boots
5) Void Staff
6) Soul Shroud
7) Lich Bane/Rylai's Scepter
8) Selling Haunting Guise at this point is fine, Nashor's Tooth, Banshee's Veil or Zhonya's Ring would be best choices
Abilities:
Barrel Roll: Your biggest damage dealer, it's your best way to kill, push, defend, and intimidate enemy characters. Upgrade Barrel Roll to Level 5 first.
Body Slam: With Magic Penetration and your short cooldowns this ability is great for jacking up the enemies but this is mainly your way of initiating fights with unsuspecting solo enemies, and keeping your distance from enemies, which you will do a fair amount of. This is your second priority for reaching Level 5.
Drunken Rage: For what it is it's fine but keep this at Level 1 until late game since you don't need the boost for Attack Power that much, and it's best for just quick mana regeneration and activating your passive.
Explosive Cask: Great for jacking enemies low on health in team fights, setting up enemies into barrels, killing enemies at their towers, escaping from groups of enemies, disorienting enemies, saving teammates, andaccidentally pushing enemies away from team fights followed by the response "THANKS FOR THE PUSH GRAGS!".
Masteries: Cooldown, Mana regen., Improved Exhaust and Clairvoyance, 15% Magic Resist Penetration. Try to have those things. They are important.
Tips and Strategies for AP Gragas:
Try to control the grass at the start: Clairvoyance it, and push in with your partner. A well placed barrel should scare off the loiterers.
When enemies are low on health at their tower early game: If Explosive Cask is available, it's your best option for finishing off heroes once they drop below 200 health, but you will probably need to dive a little for this, so it's best to clairvoyance, and then quickly body slam yourself behind the tree line to set yourself up for a throw.
Another technique you could use is to set up a barrel a good distance from the enemy hero (He'll probably assume you just misclicked), then when or if Cask becomes available, knock them back using Explosive Cask into your barrel for at least 400 damage and a well deserved "OMG" from the man you just murdered. The time barrels stay latent after a roll should give you enough time to set it up properly and you can always roll another one if it pops too early.
When pushing with your team to a tower: Rolling a barrel directly behind or next to the tower is a great power play for Gragas. The enemies by now know to run from barrels, so use that to your advantage. Scare them off, and if they get brave and want to tussle at the tower pop it right in their face.
When your team is pushing an inhibitor tower: Great time for a good gank. Clairvoyance the area around the tower and hide behind the walls. Roll barrels until someone gets close enough to the wall and far enough from their team, then set up a barrel, Body Slam into them, and blow your Cask. The only heroes that far from the tower will be the squishy ones low on health so enjoy your free kill and your safe escape 3.6 seconds later.
Best and worst getaway routes for Gragas:
Near enemy base along the walls. It's especially easy if towers are down and your are pushing, just keep swapping between the outside and the inside rim of their base.
Bottom and top zones in general is best for Gragas. Because of the isometric view it is harder to cross horizontal walls on the middle. You usually end up targetting the top of the wall instead of the opposite side. This is especially true at the Baron Nasher and the river crossing in the middle. The wall on the top-right side is near impassable.
Bottom lane is best for Gragas. There are lots of walls. Easily passable cliffs. Vantage points for barrel rolls onto the towers. Top lane just isn't as good.
The chokepoint at the dragon is a great escape route, enemies will usually follow you in and the thin walls around it make it one of the easiest getaway routes for Gragas.
Use your ult!: The easiest mistake to make when your starting as Gragas is thinking your ult isn't expendable. Even if you don't have the greatest angle on a whole enemy team during a fight throwing 1 or 2 enemies back at your team is better than bowling for strikes constantly.
Don't be afraid to use it for easy kills either, tossing your Cask is easier and more effective than trying to time a good barrel roll. (This will probably be the thing that gets you the most "THANKS FOR THE PUSH GRAGS" comments. Don't let that disparage you. If anything take it as a compliment for being such a gentleman.)
Notes on Specific Heroes:
When fighting Blitzcrank: Attract his attention, nobody can dodge grabs more effectively than Gragas with his Body Slam ability. Grabs are easy to predict and Body Slam has no startup; you haul ass the second you click the screen.
When being chased by Nidalee: Be careful of placement on your ult if you are going to use it to knock her back. Her pounce acts as a flash, and timed the wrong way, it won't do any damage, and won't knock her back or forwards at all.
When fighting Heimerdinger: Try to lane against him in the beginning. He is the easiest target for Gragas (And really everyone), and barrels can drop turrets quickly if he has them clustered in the bushes.
When fighting an escaping Twitch, Eve, or Shaco: Remember that Body Slam will hit invisible enemies, and it will still slow them. Especially against Shaco, it can help greatly with catching up with him if you guess correctly on the direction he blinks. You'll know if you make contact, and thats a good indication of where to roll a barrel or use your ult for a kill.
When low on health, fighting against a Karthus: Don't rush Karthus to begin with. He destroys enemy champions with his AoE and there's not a whole lot for you to do once your face to face with him. If you are laning and low on health, spam abilities and proc your passive. It may just heal you enough to protect you from his ult.
When Nunu is ulting during a team fight: Explosive Cask won't knock him out of his ult, but it will push him away, and also move the AoE circle for where his ult will hit. It can be a bit too risky, and someone with a stun would be better equipped for dealing with Nunu, but if you think you can clear him away go for it. Otherwise you might just throw him into a teammate and give Gragas a bad reputation.
When playing against a Teemo or AP Shaco: Barrels can be used to clear spots you have seen Mushrooms or Jack-in-the-Boxes.
When the enemies are pushing your tower: Try to avoid using Cask to launch tank characters out in front back at your team. You are only helping them do their job better because your team is likely to waste all their crowd control and attention towards that character now.
When laning against Taric or Soraka: Never lane against Taric or Soraka.
Remember, it's not necessarily the build that matters for AP Gragas, it's how you use his abilities to your advantage. Gragas has ways of setting up all 3 of his damage dealing abilities into one giant, floppy DPS burst, so long as you know when to use them and how.
Summoner Spells:
Clairvoyance, Exhaust
Exhaust: Once you blind any DPS Characters you are guaranteed to land most of your abilities on them, and either they will run away from the burst, or they will die from it. Your fists weren't meant for fighting, they were meant for rolling. Remember that.
Clairvoyance: Manna from heaven! This ability will score you plenty of ganks mid and early game, and it's usefulness for the whole team late game is obvious. Great for keeping people out of bushes and getting easy barrel rolls too.
Item Build:
Focusing on Magic Penetration early is a good investment for later, and Rod of Ages will cover you on the HP, Mana, and Ability Power fronts well past the time you buy Void Staff. Only real problem with this build I've found is a lack of Mana Regeneration especially early game. Haunting Guise sometimes isn't quite enough.
1) Rejuv. Bead+Faerie Charm ---> Haunting Guise
2) Boots
3) Rod of Ages
4) Boots ---> Sorcerer's Boots
5) Void Staff
6) Soul Shroud
7) Lich Bane/Rylai's Scepter
8) Selling Haunting Guise at this point is fine, Nashor's Tooth, Banshee's Veil or Zhonya's Ring would be best choices
Abilities:
Barrel Roll: Your biggest damage dealer, it's your best way to kill, push, defend, and intimidate enemy characters. Upgrade Barrel Roll to Level 5 first.
Body Slam: With Magic Penetration and your short cooldowns this ability is great for jacking up the enemies but this is mainly your way of initiating fights with unsuspecting solo enemies, and keeping your distance from enemies, which you will do a fair amount of. This is your second priority for reaching Level 5.
Drunken Rage: For what it is it's fine but keep this at Level 1 until late game since you don't need the boost for Attack Power that much, and it's best for just quick mana regeneration and activating your passive.
Explosive Cask: Great for jacking enemies low on health in team fights, setting up enemies into barrels, killing enemies at their towers, escaping from groups of enemies, disorienting enemies, saving teammates, andaccidentally pushing enemies away from team fights followed by the response "THANKS FOR THE PUSH GRAGS!".
Masteries: Cooldown, Mana regen., Improved Exhaust and Clairvoyance, 15% Magic Resist Penetration. Try to have those things. They are important.
Tips and Strategies for AP Gragas:
Try to control the grass at the start: Clairvoyance it, and push in with your partner. A well placed barrel should scare off the loiterers.
When enemies are low on health at their tower early game: If Explosive Cask is available, it's your best option for finishing off heroes once they drop below 200 health, but you will probably need to dive a little for this, so it's best to clairvoyance, and then quickly body slam yourself behind the tree line to set yourself up for a throw.
Another technique you could use is to set up a barrel a good distance from the enemy hero (He'll probably assume you just misclicked), then when or if Cask becomes available, knock them back using Explosive Cask into your barrel for at least 400 damage and a well deserved "OMG" from the man you just murdered. The time barrels stay latent after a roll should give you enough time to set it up properly and you can always roll another one if it pops too early.
When pushing with your team to a tower: Rolling a barrel directly behind or next to the tower is a great power play for Gragas. The enemies by now know to run from barrels, so use that to your advantage. Scare them off, and if they get brave and want to tussle at the tower pop it right in their face.
When your team is pushing an inhibitor tower: Great time for a good gank. Clairvoyance the area around the tower and hide behind the walls. Roll barrels until someone gets close enough to the wall and far enough from their team, then set up a barrel, Body Slam into them, and blow your Cask. The only heroes that far from the tower will be the squishy ones low on health so enjoy your free kill and your safe escape 3.6 seconds later.
Best and worst getaway routes for Gragas:
Near enemy base along the walls. It's especially easy if towers are down and your are pushing, just keep swapping between the outside and the inside rim of their base.
Bottom and top zones in general is best for Gragas. Because of the isometric view it is harder to cross horizontal walls on the middle. You usually end up targetting the top of the wall instead of the opposite side. This is especially true at the Baron Nasher and the river crossing in the middle. The wall on the top-right side is near impassable.
Bottom lane is best for Gragas. There are lots of walls. Easily passable cliffs. Vantage points for barrel rolls onto the towers. Top lane just isn't as good.
The chokepoint at the dragon is a great escape route, enemies will usually follow you in and the thin walls around it make it one of the easiest getaway routes for Gragas.
Use your ult!: The easiest mistake to make when your starting as Gragas is thinking your ult isn't expendable. Even if you don't have the greatest angle on a whole enemy team during a fight throwing 1 or 2 enemies back at your team is better than bowling for strikes constantly.
Don't be afraid to use it for easy kills either, tossing your Cask is easier and more effective than trying to time a good barrel roll. (This will probably be the thing that gets you the most "THANKS FOR THE PUSH GRAGS" comments. Don't let that disparage you. If anything take it as a compliment for being such a gentleman.)
Notes on Specific Heroes:
When fighting Blitzcrank: Attract his attention, nobody can dodge grabs more effectively than Gragas with his Body Slam ability. Grabs are easy to predict and Body Slam has no startup; you haul ass the second you click the screen.
When being chased by Nidalee: Be careful of placement on your ult if you are going to use it to knock her back. Her pounce acts as a flash, and timed the wrong way, it won't do any damage, and won't knock her back or forwards at all.
When fighting Heimerdinger: Try to lane against him in the beginning. He is the easiest target for Gragas (And really everyone), and barrels can drop turrets quickly if he has them clustered in the bushes.
When fighting an escaping Twitch, Eve, or Shaco: Remember that Body Slam will hit invisible enemies, and it will still slow them. Especially against Shaco, it can help greatly with catching up with him if you guess correctly on the direction he blinks. You'll know if you make contact, and thats a good indication of where to roll a barrel or use your ult for a kill.
When low on health, fighting against a Karthus: Don't rush Karthus to begin with. He destroys enemy champions with his AoE and there's not a whole lot for you to do once your face to face with him. If you are laning and low on health, spam abilities and proc your passive. It may just heal you enough to protect you from his ult.
When Nunu is ulting during a team fight: Explosive Cask won't knock him out of his ult, but it will push him away, and also move the AoE circle for where his ult will hit. It can be a bit too risky, and someone with a stun would be better equipped for dealing with Nunu, but if you think you can clear him away go for it. Otherwise you might just throw him into a teammate and give Gragas a bad reputation.
When playing against a Teemo or AP Shaco: Barrels can be used to clear spots you have seen Mushrooms or Jack-in-the-Boxes.
When the enemies are pushing your tower: Try to avoid using Cask to launch tank characters out in front back at your team. You are only helping them do their job better because your team is likely to waste all their crowd control and attention towards that character now.
When laning against Taric or Soraka: Never lane against Taric or Soraka.
