My advice: Skip everything else. Just use pistol, maybe use the quick shot glitch (I found I could do it with the mouse wheel) as mentioned below and save up 8K. After that it's smooth sailing forever. I got to level 112 while watching a movie. Having the plasma rifle was pretty much win. I could have kept going literally forever, but the movie was over :-D and other than mindlessly shooting things, there doesn't seem to be much point. So I got bored and quit.
The enemies get slightly stronger at like 70 and require an extra plasma to kill. Otherwise it was the same enemies (black hood walkers, grenade launcher flyers, and UAV) over and over for 40 something levels. It wasn't really a challenge at all (they would rarely even hit me). At that point the author should have left all the enemies in and had them keep scaling to provide some kind of end (i.e. needing one more shot every 10 levels beyond 50 or something). I would have found lots of bomb bots hard to deal with maybe.
Tl;dr all you need is the plasma rifle for 99% of the medals, don't bother with anything else. Good game for what it is.