not as op as instakilling any unit that isn't a giant, but still being able to do 1/3 of a giant in, or making someone lose control of their unit, have it drawn toward the enemy castle, and making it impossible for them to retaliate where they will be slaughtered by the enemy army.
Seriously though, often, when i try to cast some offensive spell with the magikill, he will get stunned by the enemy giants, stay there, and die before the animation is complete.
more often, before he casts his poison or explosion, the enemy will have already walked away out of range, where they will return to kill my exposed magikill that died and wasted 50 mana for some pretty fireworks.