IMO Xah'rith's not really all that difficult, just keep an eye on the ground and stay out of the patches as much as possible. The one who seems out of whack to me, difficulty-wise, is Nekarat in Act 4, if you are a melee class. If I get hit by his fire patch ability even once (which he casts nearly instantly right underneath him), it can take me from 100% to 0% life WITH a potion being used, with 900 fire resist. Thankfully I got the plans from him after the 10th kill or so and so I don't have to kill him ever again, but still...
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