I think this is the best build for any AP caster with AoE magic damage. To make this build work, you will need to start team fights as early as possible to get stacks on your mejais soulstealer. The principle of the build is that the only really needed item for AP on Gragas is the soulstealer. This, along with his dorans rings, will give you about 300 AP at max stacks. The reason i use a snowball item on gragas, is because he has high burst damage so he can pick up kills easily. He also has AoE damage and can pick up assists. For this build, getting assists is almost as important as getting kills, as they will make up most of your stacks on the mejais soulstealer. Immediately after getting the soulstealer, build a giants belt to turn into a warmogs. the last two items are situational. If they have a heavy AD carry suck as Tryndamere, or Master Yi, then pick up a thornmail. If they have a heavy AP team, or just a fed AP carry, pick up a Force of Nature. With max stacks on your soulstealer, you should have a lot of AP built up, and this is why my last item in the build is lich bane. A body slam auto attack combo with lichbane and Drunken Rage does a TON of damage on its own. Followed by his barrel it can nuke down enemies to 1/4 health, and you can finish them with Explosive Cask.