The only way i see it as not to one sided would be a rock, paper, scissors
Something like have the players press a direction when they start the headon and the thing they are holding 1 second after the headon started gets read is the imput.
They would go something like forward beats back, left/right beats forward and back beats left/right.
Forward would be a push forward.
Left/Right would be a flanking try.
Backwards would be a jump back and attack.
Basically forward would catch the jump backwards and deal some pain, flank would evade the forward push and hit the side for some pain and back would evade the flanking and attack.
The different colour cyrcles are the characters, the arrows are their movement.
So yea thats the gist of my idea for headonns XD