Yes it is. Stone skin provides a great boost to physical resistance. God armor provides a nice boost as well. For me, Target + Stone skin got me through impossible as a Mage without losses.
You should be aware that on normal, the stacks are not so big so they aren't that dangerous. On impossible, for example, Elenhel on lvl 40 had 115 necros, over 300 druids, 187 (or was it more) archmagi, 250+ demonologists. Each one of them deals magic damage, and his geyser usually kills 1/6 of a warrior Amelie's stacks, 1/5 of paladin amelie's stacks and 1/4 of a Mage amelie's stacks in one go - and he can cast 6 of them without thinking.
You can imagine what kind of firepower is that - in both physical and magic damage.
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