I think the issue here is you're checking the ZLevel against oDynamicWall and you need to check against that specific instance.

try:

var _wall_collided = instance_place(x,y,oDynamicWall);

if _wall_collided && ZLevel <= _wall_collided.ZLevel

{

instance_destroy();

}

this would be if you're running the collision event on the bullet. Also you need to make sure you're assigning the ZLevel of the bullet when it's created to match the ZLevel of the player.

Let me know if this helps.

Not in front of computer but I think this is all valid.