[GetThere1Time]

ok here's a few strategy points

I went no damage -> faster soldiers -> no power bar -> take no damage -> double shot -> quick shot

If you do this you should be fine for a while. It'll get hard at around lvl 20 before you get double shot but if you can tough it out you're golden

Some pointers:

-Aim for the head
-Get close to the enemy. I usually go around 3/4 acrossed the screen.
-Even when the screen is clear keep firing off of it.
-If you start to get swarmed get right next to the baddies and walk backwards. While walking backwards aiming toward them at a 45 degree angle. If you're being chased by a horde all you have to is tap the button since they'll be right next to you and you wont have to worry about charging up the power

hope that helps