What exactly is the cleric having troubles with? I may be able to give some tips...
I don't know how far they've gotten the health down, but if you say quite a ways it makes it seem like the cleric may actually have the strat down. It might not be their fault, the dps can mess up that fight too. If the dps aren't jumping or doing an action to get out of invulnerability after rez fast enough, then the boss will use Midnight Wing, and that makes things a lot harder than when he does Threshing Wind. Don't know if that's the problem, but that'd be my first guess.
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