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I would like to say there is a shorcut or "cheat" for this but, there isnt. The trick to this level is memorization. Go into free play and load up Area 2 by itself. Play it enough to learn where the enemy waves come from and fire shots before they arrive in order to maximize your time before they shoot. If necessary you can even write down the order they arrive in. This achieve is just a simple, try until you can beat it. Nothing else to it