The fourth boss is a serious pain, and to my knowledge there hasn't
been a foolproof method found of beating him. The best way I've
seen/used is to lure the boss back until he won't walk to the left
anymore. Walk back to the right and he should conjure a double behind
you. If the wizard and his double match your movements as you walk to
the right after that, you're in business (and if I remember he does
so quite often, if not 100%). Dodge what they throw at you as you
walk to the right, and when you've pinned the right wizard against
the stairs you can kneel and punch quickly. He disappears and
reappears, but he can't go anywhere because of the stairs so he's
dead in a few seconds.
As you noted, kicking off their heads does nothing to them. You have
to use low attacks to damage this particular boss. This method is
hardly foolproof, but it works often enough to be better than rushing
in and attacking. Hope it helps.