I probably should bump it up a tier, (I played it back in May so it wasn't as fresh in my memory). It's a really hard game. My high score was 25,650, playing on Difficulty 5, using shields, time warp, and penetrating shots, (but no guided shots)
I always activated time warp right away to take out the mother alien on the first shot, and then every shot thereafter needs to take out at least 2-3 at a time. Don't use the shields until the time warp is finished.