This is my fisrt guide so I hope you like it and that it isn't to bad.

General:

You use Barrel Roll to farm and to hit the enemy and bring him to lower than the half of his health. When you are level 3 you first use Body Slam to close the gap and then Barrel Roll in front if him when he triest to walk away and just basic attack him in between. When you are level 6 you can first do Explosive Cask in front of him and then Body Slam and Barrel Roll. Use Drunken Rage when possible so you don't run out os manna.

Team fights:

When a team fight is going on you first use use Drunken Rage and then Explosive Cask behind them, make sure you hit the carry with it then he/she is mutch easier to kill. then Barrel Roll in to the group (or carry) and then Body Slam in the group (or carry). use Barrel Roll and Body Slam when posibble. wWhen an enemy dashes or flashes over a wall, you just follow him with Body Slam.

Gragas Explosive Cask is very usefull in team fights. You can knock them towards you to kill them easier.

You can farm easy with Barrel Roll. When its at max level and with a few items like: Athene's Unholy Grail you can easy kill the last three minions with one hit from Barrel Roll.