Try spawning your enemies 1 at a time instead of all at once. Enemies can be spawned using a trigger zone. In the enemies' properties, set it to NOT spawn at start, set 'max at any time' to 1 and 'maximum spawn' to 1.
Place your trigger zones so that as you play through the level, each enemy is spawned individually, but be carefull where you place the trigger zones, so that you can't see the enemy being spawned. E.G. place a trigger zone before you reach a corner, with the enemy appearing around the corner where you can't see it spawning.
Hope this helps.