First add event: Player - On Collision with Enemy, then add a sub-event: Player - Platform is falling, add another condition: Player - Compare Y (put comparison: < less than, Y co-ordinate: Enemy.Y) -> (add action) Enemy - Destroy, (another action) Player - Set Platform vector Y to -500(this makes your player bounce when you jump over and kill your enemy, mario style).
Right click on the left side of player - platform is falling and compare Y, Add -> Add else.
System - Else -> (add action) Player - subtract 1 from health, (another action) Player - Set animation to "your damaged animation".
Also go to your player animations - your damaged animation properties and set Loop -> No, so you animation doesnt repeat over and over after you collide with enemy once. Instead it plays once per collision.
This should look like this...
I hope all this isnt to much complicated.