Oh? I was under the impression that Berserk for enemies is literally the same thing as Atk up. So +0.5.
I paid attention to this. With 45% Atk Down, Tiamat did 450ish*3 with a special attack, then got herself Berserk, then did 900ish*3 with a special atk.
So, just like Atk Up, Berserk is just added to the modifier.