Finally beat challenge 6. My god, what a pain it was. I could get to wave 5 with little issues and be left with the bogeys. But for some reason I was having a hard time dodging the straight laser which normally was the easiest to dodge. I would have full rockets, 5 emps, 1 upgrade and 1 lock on laser. More then enough to beat the challenge, but I kept screwing up. I must have spent +40 hours trying to beat it and in the end beat it in under 21 min.
Wave three is very easy if you know this trick. The area where you can get the lock on laser in the middle of the field is a safe point where for some reason none of the enemies will react to you. Before the round starts hide there and shoot the lfe at the spawning gorgies and you should be left with only romanovas. I kept getting hit by the shoulder laser from the left romanovas until I positioned my self to blow it off. From there you can safely shoot the romanovas by shooting of their heads or killing them out right. For some reason that phase use to give me the most trouble as well because the key to most phases is to wipe out the minor enemies as quickly as possible and that phase had the most number of tough enemies.