Tyrannical 14 Upper karazhan
3rd boss: got it to 3%. The orbs do 2Million damage per stack every 3 seconds and the blast from the boss that hits everyone for 2.5Million damage
Also Medivh is just nasty, if you get double inferno blasted you die, and arcane missles hit dumb hard. would not bother without 4 interrupts.
Explosive Teeming 13 Arcway:
The last pull before Zakuun. Each Inquisitor will cast their prophecies of doom randomly. Many times seconds into the fight, doing 2.3M damage in quick succession, wiping the group.
Had to wipe about 7 times, killing the one cast, then heroism and hide behind wall and kill the 1 inquisitor luckily on a pull.
Cathedral +10 anything
Bosses are just dumb and do too much damage, mostly 3rd one and 1st one.
Darkheart Thicket Explosive
The little adds that sprout on Oakheart create explosives too, all over the room and many of them.