just played a bit of xenophobe, the rifle is best because it can be aimed when using the jetpack whereas the laser gun can't.
I usually keep the character crouching and shooting, wait before entering a door, until I don't hear any xenos kill sound anymore.
To gain some time, when there's no xeno in a room, I make a jump to speed up a bit and then crouch again.
For the last level, keep a lot of bombs, kill all the xenos then use the jetpack to fly just above the rolling xenos and shoot the queen down.