Try and time your innervate where you can get it off twice throughout the fight. Also that flask that regenerates mana helps quite a bit and isn't very expensive. If you know you can't kill the channelers then just save your mana for Mags.
The damage you take varies each fight because of the ceiling falling, number of adds getting off the tank, and if you're in the wrong place then you take a big hit and waste a ton of mana.
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