I actually try him on H, but the ball is the same thing.
I prefer to chose Dampen Harm on the 8th, 9th and 10th stacks since this CD is always up (Diffuse Magic works, obviously).
Try to always stacks 10, and to just stack the ball on him about 1-2 secs before enrage since you'll gain time on berserk.
You can survive to the enrage, really. You can keep your FB for this to help a lot.
How I do it with Dampen Harm :
First ball : 10 stacks = Guard at the beginning of the fight to have it for the 5th stack. Dampen Harm for 8th, 9th and 10th stacks.
Second ball : 10 stacks = FB at 5 stacks and Dampen Harm for the 8th, 9th and 10th stacks.
Third ball : 10 stacks if you have problems of dps = Dampen Harm when damages become high. (8th, 9th, 10th stacks if possible).
Fourth ball : 10 stacks if you can = Dampen Harm for the 3 last, as ever, but no FB. More, you can kite the boss to squeeze dps and reduce enrage survival timer.
Enrage : Immediatly use FB while he starts attacking, you should have 20 secs of survival which allow you to destroy the boss.
Ball ticks for 450k at the 10th stack on Heroic, I don't remember on normal difficulty but it appears as something like 250/300k.