Best thing:
use the events event_impact and event_entity of the enemy and check the z value of the you entity:
Code:

---

function enemy_event()
{
  if(event_type == event_impact || event_type == event_entity)
  {
    if(you == player)
    {
      if(you.z > my.z) { my.health = 0; }
      else { you.health -= my.damage; }
    }
  }
}

action enemy_act
{
  my.enable_impact = on;
  my.enable_entity = on;
  my.event = enemy_event;
  
  my.health = 100;
  my.damage = 5;
  
  while(my.health > 0)
  {
    // do whatever
  }

  //Die part:
  wait(1);
  ent_remove(me);
}

---