Ment
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I also agree with Heroist.
Regarding Tree, perhaps she should cast a spell to reflect damage onto an ally (who will "hold" the ghost, for example), or start healing. For now, the Necromancer himself seems to be safe. By the way, there's a chance that her beam of light will damage the ghost.
In other words, if you position yourself correctly, you can kill two birds with one stone.
In Lokken's place, I would definitely use purification on the ghost. The skeletons don't seem as dangerous as I thought. Or you can try using shackles...
In any case, this ghost is the most worrying thing right now, so we need to focus on him.