Nuking bosses with spells oonly works on normal. On hard and impossible you run out of mana before you even manage to get the boss' helath down to 50%.
I still wonder how did N3MES1S fail to beat them up when he had stone skin all the time. As a mage, he can easily get 2 units close to the boss, stone skin them and hack. That alone is enough as a mage to beat the boss on impossible (both with and without losses).
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