Its called a soft-enrage, its a race between the team killing it before Healer run out of MP. The softer are just spammed to no end, the harder one get stronger every time until none is left.
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Tanks also have shields/reprisal that can be used, Ranged have their party wide mitigation skill, and Casters have addle to make it easier on the healers. They're just not frequently used in DF. Once gear levels increase it won't even matter as much as the dps will be so high she'll only get to cast it once or twice. I was already in a group where Shiva was 5% before it started and didn't get many casts off.