Impaled :
Kill an enemy in mid-air with a stab. It has to be a regular fall, not a jump (or a jump from somewhere considerably higher than where you are)
SPARTAAA (can't believe this isn't in yet) :
Make an enemy die by fall damage with a kick
Mike :
Kill an enemy with a punch, a kick, a nudge or melee stones
Let me show you :
Kill an enemy with his own weapon (has to be the exact weapon he previously used)
Exactly as planned :
Kill an enemy with a backswing (early overhead catched by the person behind you)