How do you get berserk on Kingdom Hearts 2?

Answer 1

Why, that's easy! If you're talking about the Nobodies, then listen closely!

If you ever come across a Nobody that is armoured and has reasnobly large life, attack it from behind. When it drops it's anchor-like weapon, go up to it and use the Reaction Command, Berserk. Float up to the Nobody and Use the Reaction Command Eclipse for massive damage to be taken to the Nobody!

However, if you mean the ability, it depends which Dream Weapon you chose as Roxas in the Beginning ie. the Dream Sword, the Dream Shield or the Dream Staff. You get it at certain level ups.

but if you defeat the goddess of fate cup in kh2 in the underworld you get a keyblade that has 3 strength and 5 magic and its keyblade ablility is beserk