Just to add he does it at 33% aswell unless he does the strangulate before but pretty sure he will try to use it as soon as you interrupt his heal. Also he uses not once but every time his life reaches 50% and 33%, which means if he aoe heals and his life goes above 50%, he will try to mend again when it reaches at 50% health.
For boss 4 you are right, appart from all the aoe he does, he has got a nasty poison damage which seems to be a small aoe which affects anyone around the potion's blast radius. He moves from place to place but very slow. You have enough time to position around his room to avoid getting more than one person to take the poison damage.
For Boss 5, you can let the debuff wear off when he is summoning the ghouls or doing the barrage. Just get away from him a bit till the debuff wears off.
I would say appart form boss 3, SFK is doable on any group configuration. VP I agree, the 2nd boss could be a pain in the @ss on all melee group, but since I have no exp on such group config I can't tell.
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