When you get 100 stacks of Corruption, all your casts are instant and deals 100% more dmg..
You're immune to healing though so you won't survive for long. But if you have 10 ppl alive in your grp and no combat res used, get some shields and dmg reduce effects rotations on that mage and he can do a shitload of dps for some time. Then combat res him and no harm done..
So ye, they got him to 100% corruption and then he just spammed Pyro.