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 ur death easily.
I would follow this steps to do it easily:
1. Keep the boss surounded by 4 pilars

2. Use the galthen's chestplate to paralize the boss

3. Use the mirrors of the 4 pilars around the boss so they face it (you can do this before of after using the chestplate, but be fast because they change by themselves sometimes)

4. Get out the UE area aroound 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 repit all the steps everytime the chestplate appears until it dies.