Add a layer above Main and move your object between layers as appropriate?
You'll need an event to keep track of when your guy is in front or back of the enemy. Construct is a 2d platform and you're trying to do 3d or at least 2.5 so you'll have to get creative.
+ Every tick
- if player.y < enemy.y then enemy is in front of player (switch enemy to new layer)
- else if player.y > enemy.y then enemy is in back of player (switch enemy to overlay layer)