^ I couldn't have put it better, ZicherCZ. :)
This was a problem that plagued me when I was going for Destroyer Diamond. (Before I realized that there's a bug that makes it impossible to get, even if you fulfill the conditions) I must've restarted the game about a hundred times; level 1 was definitely the biggest hurdle to success. Once you've eliminated everyone outside the structure, the rest is a relatively simple case of cornershooting, preferably with the Shotgun, but those guys on the outside at the beginning can come from any direction, and frequently multiple directions, and those trees really don't offer the best cover. (Though they're better than nothing; I've used the same trick of running Former Humans out of ammo many times) ;)
Considering all this, I really don't get how 2Dev can claim to be able to consistently play damageless games. :|