Well.. P8 hell baal has 1 727 953 hit points .. and 50% physical damage reduction.
A little attack rating calculation:
You are level 80, with an estimated 6000 attack rating, against level 99 baal which has 2847 defense AND 55% chance to block.
Using a simple attack rating calculator, this equals to 27% chance to hit.
So say if you deal about 5000 damage on average including critical strike, and do about 4 attacks each second, and only about 1/4 of those hit (27%), that damage is reduced to 2500 each second.
1 727 953 / 2500 = 691 seconds.. that's over 10 minutes. I'm probably a little bit off.. but it gives a vague picture.
It also speaks for itself, crushing blow is the #1 rule to deal with such high hit point targets. You have any cb at all? Guillaume's face alone would help a lot i imagine... maybe both on you and your merc.