I simply can't kill that last boss (Keristrasza). And my team is even level 76 - I have been trying forever now.

Not sure im doing it right - I just position myself infront of it and opens with Flame Shock + Lava Burst. After that I just cycle Lightning Bolt / Chain Lightning and I try to get a Flame Shock and Lava Burst in. But after roughly 30-40sec my shamans get REALLY low in hp and very fast. Grounding Totems are up and I remember to jump every 2-3 debuff stacks. I am trying to heal through it - but the damage is just to heavy... Using Chain Heal and Lesser Healing Wave (for the low cast time). It's normally around 50% the boss goes balastic on me

All the Shamans are identical:
Elemental specced 62/5/0
11193 Health
12136 Mana
10886 Armor
+922 Spell Damage
+339 Mana Regen

Any ideas - ready to pull my hair out now