I just got this last night. I didn't think it was so hard because in the first 3 tries I had gotten down to the last bug or two right away. 300 tries later, I finally got it =/
Ironically, I've done it now a few times when I play the actual game. It is somewhat luck, especially when you are first doing it, and there will always be some luck factor in it when certain bugs don't multiply or disappear. The only helpful hints I can give are the following.
-You can always get the right or left bugs in 4 shots right off the bat. I prefer right just because your gun defaults tilted that way and feels more natural, but they never multiply on me.
-Always angle towards the middle. If you do the right side first, when you move to the left side angle towards the circle and try to keep the bugs pinned between you and where you can finish the level. This keeps them from multiplying behind you and you never have to readjust your aim, plus sometimes they just despawn if they are too close to the wall, and you will already be facing the middle for the top if all goes well. It'll save around a second of your time.
-I find getting as close to the bugs as possible helps a ton. I find I am able to fire a lot faster, I miss less, and it seems to prevent them from spawing if your body is over the explosion animation.
Whenever I try for level 12 (ha, good luck on me doing this achievement...ever) I always play the first level in a way to clear all the spyders and I've now done it a few times. It feels less luck based now that I've got the pattern down. I still screw up all the time though.
I got it!! Well, I guess I was wrong. It is absolutely possible. It just seemed really hard at first. If you hit a bug with every single shot and don't ever miss, with just a bit of luck you can do it.
It seems that I must close and lock this topic, as it has been proven that it is indeed possible.
What the devil did she bring me into this world for unless she had plenty of money to give me so that I might enjoy myself in it? - Sweeney Todd