The Chosen Ones

For sure it is possible, you just doing it in the STRAIGHT way - always attack.
All you have to do - is to destroy his mana (it is hard, but not impossible), some items and hero spells may destroy mana quiet fast. During his summonings of your heroes illusions - you have to just move back and don't attack illusions - because if you kill for example Phodom's illusion - your Phodom would instantly die.
If The Boss has some mana - it is impossible to kill him - fast regen. So 1st stage - kill mana - next stage just kill him with direct attacks.
Good luck!