His barrel roll when fully lvl'ed takes alot of mana out sometimes you might have to get a item that can give you a decent amount of mana like RoA But with his drunken rage he can resore mana decently and his passive keep him up with hp making it to where if you poke and the minion or champion him or herself hit you it wont take that much to heal up. His ult always has to be targeted right because it knocks back the champions in the Area of the explosion. Oh and dont forget when chasing or starting a fight or running away even use body slam it slow the enemy champion when hitted, can get you to them faster, and away (also can go through walls with it)

When playing gragas look to where the enemy champion is walking toward and try to angle up you barrel with the dirrection they are going, and make sure to pop it quick. Also if you start to win your lane after level 6 you can start to gank other lanes like bot for instance go down to the tri brush closest to the enemy turret and have the jungler with you and when you go in flash in body slam them if you cant do that throw your ult in front of them knockcing them back to your team then barrel roll to them and explode it hopefully securing the kill (KS= Kill secure). Late game its always good to use your ult to split the enemy team up, sometimes it makes it easier to get to the adc or whom ever you are targeting.