If you have max str you should be hitting 9s, so on your turn attack him from 2 spaces away (so the Black Knight can't activate luna) then heal Ike with mist. The Black Knight then sould attack you directly taking another 9. You attack from a distance, heal, then get attacked. If you keep doing this you should end up killing the Black Knight in 5 turns and not have to worry about a random Luna killing you.