My group stumbled head long into the Erylium encounter, (party consists of a Dwarf Paladin, Half-elf Rogue, Half-elf Ranger, and a Gnome sorcerer). After seeing Erylium summon the sinspawn, having dealt with the other sinspawn rather easily, they figured it was just another simple encounter. But after her turning invisible and the sorcerer noticing not much blood or damage when she reappeared they got pretty concerned.
So as a team effort the Ranger and Sorcerer lasso a rope around her and start to pull her in, our burly dwarf aids in reeling her in all the way, and finally the ranger succeeding in grappling her takes her over to the pool in the center of the room and proceeds to drown her. Everyone jumps in the pool to make sure she has no chance of getting out. Take THAT dmg reduction and fast healing!