could you just have the animation wait till the enemy hits the ground? isn't the platform gravity still affecting the enemy?
maybe use a boolean on the enemy 'AirDeath'.. it could stay false until
player jumps on enemy head AND enemy platform is NOT on ground -- set 'AirDeath' true
set enemy.y to enemy.y + 6 (or whatever)
then when they land, have it set airdeath back false and run the death animation..
I'm tired, and I feel like there's probably a better solution.. but maybe that helps..
if not, got a capx we could look at?