Why not randomly create the enemy together with each platform (that is allowed to hold enemies)?
Or check the position of your player (player.y) and run a enemy creation function that creates an enemy on a random platform 600 till 1000 pixels above the player (or other values).
To do so just use system -> pick (platform) by comparison and compare the y-values.
Or if enemies are to be in the air, just let the system create an enemy and set their locations in relation to your players position.
like x: player.x+floor(random(400)) y: player.x+600+floor(random(400))