Because Mizod is a Necrotic Leech, he will hurl necrotic balls at heroes. In addition he has a "Damage Reversement Aura", that will heal him, while other Minions and Lieutenants are damaged around him.
Because he's a Captain, he has additional suffixes: Spiky and Spiky Aura, which will make physical damage backfire at melee heroes. So to fight Mizod you'll have to use ranged heroes. As a bonus, Mizod is annoyingly fast, making homing attacks the best and/or slowing/immobilizing attacks like Flak Cannon, Strangling Briars and Gravity Storm. However, before you attack Mizod, make sure you killed all other enemies, 'cause otherwise (if the enemies are near him), whenever the Minions/Lieutenants recieve damage, Mizod will heal, making the fight pointless, unless the other enemies are dealt with.