So... I must confess I was the guilty of killing half my team in the boss room XD
It is not hard at all to do, but if you don't know how it can be your death easily.
I would follow this steps to do it easily:
1. Keep the boss surrounded by 4 pillars
2. Use the galthen's chestplate to paralize the boss
3. Use the mirrors of the 4 pillars around the boss so they face it (you can do this before or after using the chestplate, but be fast because they change by themselves sometimes)
4. Get out the UE area around the boss (he can deal over 8k hp)
5. Throw one GFB/Avalanche/rune to the boss, this mean that only 1 of the team uses it every time you do this sequence. When you do this, he reflects the damage in an ue
6. Don't attack it while it moves because you heal if, and repeat all the steps every time the chestplate appears until it dies.