Ok, i think i have a thing for underrated champs and it really shows due to the fact that I LOVE Gragas! His barrels will take a squishy or a tank down like a BOSS if you use my build. Other builds won’t give you enough penetration which ends up in a beast early game but you won’t be able to do **** in a late game. Sooooooo in my build I put in a pretty decent amount of magic pen as well as AP and some alright MR and Armor.

For runes I grab some early health with Quintessences which will help him survive more because at first he is pretty squishy. Next i grab The greater mark of insight for some smooth MP. I get this for some early AP to help out my barrels. Next I take some armor in case I find myself up against a pro yi xin, or tryd because these champs can slaughter grag if they get close enough. Although your barrels should be able to do some serious damage if you are diligent enough with them.

These masteries are good throughout the game as this build will help you do some damage without dying. I like the AP they give you as well as the health, armor, and MR that these masteries help give you.

First off, Doran's ring. This item is essential to grag as it has all the stuff he needs at first, Health, AP, and mana regen. Even with mana regen from this item you will still need clarity to do stuff at the beginning. But this item gives you the good start you need with grag.
Next are your boots. I put Mercury's treads up there but if you are fighting an attack heavy team by all means go for the Ninja Tabi . But if you have a choice go for Mercury's.
Next up the RoA . This is a very good well rounded item that is just a plain good starter. It gives some much needed health and AP. Remember, if you are building a item ALWAYS GO FOR THE HEALTH PART OF THE ITEM FIRST! in this case it would be the Catalyst the ProtectorCatalyst the Protector.
The banshees is for more health, good MR, and a sweet passive that really helps grag.
Next up is the Zhonya's Hourglass . This item isn't SUPER useful due to the fact that fifty armor is not very much and the passive is not SUPER effective because its gragus's job to stay out of teamfights and chuck barrels in. But he needs the AP so i couldn't switch it for a frozen heart or somthing like that. If you want health instead of armor, feel free to go for Rylai's crystal scepter instead, because its passive actualy does a pretty good job with your barrels and it works pretty much just as well.
Now for some butt whooping. Time for Rabadon's! This is just for plain nice AP to help your spells do more.
If the game is still going by this point, sell your ring and grab the Abyssal Scepter for nice AP, MR, and MP.

For skills a lot of people say not to level up your barrel. I say LEVEL UP THE BARREL! The barrel is your bread and butter and you need to be constantly aiming it quickly but smartly at enemy champs. You will harass the Poo out of them and will force them to turret hug. Very effective if you are with a squishy champ who needs to farm off minions at first.
Your Drunken Rage is also very effective for regaining mana and always remember to use it before going into a fight, it has very good defense.
Body slam is also very important due to its crazy short cooldown and long slows. It can effectively be used for chasing and running away and it can go through some skinny walls. This can also be used to harass if you jump into minions and there is an enemy champ nearby, due to its splash. Before I had you level up Body slam along with the other spells, but thanks to some good advice from Toshkun I have decided to level up drunken and Barrel Roll instead. He said, " I suggest that you only keep 1 level of Body Slam at lvl 2 or 4 and max Drunken Rage second because bodyslam is the same cool down all levels."
His ult, Explosive Cask , is VERY GOOD and has HUGE potential. First of all, its huge range and big damage might give you an unexpected kill if you go to the bushes, body slam forward and chuck it at a recaller who underestimates the damage it does. It is also very good it team fights, but PLACE IT CAREFULLY! If you place it right you could icolate a yi , trynd , noc , or udyr , or any other tough champ who is giving your team a hard time and destroy them before the rest of the team can recover. Or it can be used to save you or a friend from certain doom if you or they is being chased by like three champs. It has limitless potential and a lovely low cool down so don’t be afraid to use it.

Now I used to say that, "Gragas needs clarity". But Ive officialy changed my mind and decided that ignite is the better option. With ignite, your whole spell chain, and a well placed ult, you can take out their ad carry pretty fast.
I take flash because it helps a bunch with running or chasing, and grag is good at both. I used to get teleport but Gragas really doesn't need it too much, he needs to stay out as long as possible so tele won't be too much of a help.

When you play Gragas you need to know the champs that can slaughter you, and sadly there are quite a few. Here are some not to engage 1v1 unless they suck **** or u are rocking beast. Now if you are with your team these champs are your specialties. Simply wear them down with your barrels when your teams are circling, and when you have about a quarter to half their health gone body slam in and separate their team with Explosive Cask .(you should already have used Drunken Rage ) Try to place it where you will send their tanks away and bring one of the champs listed above to your team, but if you just want to confuse and separate them go ahead. Other then that if you are laning just stay in bushes and shoot out barrels at enemy champs every time you can. It will keep them back and wear them down, as well as allowing your teammate to farm minions.

Farming is Easy and effective with Grag. Just chuck a barrel in the middle of creeps. If u don't have enough AP yet for that to kill them then go and body slam the and they should all be dead.

Incredible harasser. Great ult. Super low cooldown on Body Slam.

Gragas is very good at harassing and thanks to his Barrel Roll he is actually REALLY good mid! I have had countless games mid with fatty where people totally underestimated me and i got first blood. Barrel Roll just takes so much out of squishy people it's not even funny. And its range is amazing and the area is just as good. If you go mid harass the **** out of the poor, poor karthus who has to fight you, and the whole team will fear you. I have already discussed most of the strategy of the sides. Don't be afraid to 1v1 a squishy if you have your ult . Just Chug some Juice , Body slam into them, chuck your barrel , then when they are getting away ult them back to you and body slam them again. They should be dead by now but if you need to use flash to get another bodyslam\ barrel roll do it and they will die. One more thing, when you are picking your champion with your team make sure they know you are AP gragas and someone else needs to tank. You really will be a half tank and not totally AP but it never hurt anyone to have too many tanks -.-

Never tried juggling with him although i think it would be very good due to his body slam, Barrel Roll, and Drunken Rage. Maybe I will make a Juggling Gragus Build. Vote this up and comment if you want me to.

Pros
-Great range
-Great damage
-Huge harasser
-Great ult
-Good in team fights
Cons
-Squishy
-Squishy
-Squishy