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Ninja (attack with poison (each attack is accompanied by the application of some harmful spell of levels 1 and 2 from the schools of Death, Order, and Chaos))
Reaver - Rogue (bloodlust (Magic Mirror))
Ninja - having a beholder as a hero is great - it's a shame that it's only in Equilibrum :)
Rogue - again, the shield of tactics gives a shield - honestly, magic mirror is good, not like bloodlust :)
Added 4 minutes later
In this tier list, the Witcher is the best for me, as he can level up on living neutrals even without obtaining him.