How to win (kinda cheap)
(this is Defense mode)
For your first upgrade, take the G36 assault rifle. You have to get lucky--you will either get just over 350 experience, or just under it. You might as well restart if you get just under 350 experience since it's tough to last the next round without taking serious damage.
Then survive with the G36 until you have 3000 cash. This will take some serious doing and you will just barely survive the last level that takes you over 3000, because of those damn clawed thingys that are tough to kill with the G36. Save only when the round seemed to have gone pretty well and you didn't lose much health.
Now... GET THE LASER CANNON.
It should now be easy to survive. Try to line up several enemies in one shot. You need to live 2 more rounds with the laser cannon, until you have enough money to buy...
OVERKILL.
Overkill + laser cannon = huge experience because you do 100 damage with each shot which puts you way over the amount required to kill. I got over 50,000 experience the first round using overkill + laser cannon, and I spent it all on upgrades and improving the base. I just finished the second round of laser cannon + overkill and I have:
64333 exp
all upgrades
1222/1225 health
4825/4850 shield
This is now kind of silly... there's just about no way I can die.
Reviewing the game:
It's a pretty good game, kind of one-dimensional. The excessive blood that never goes away is a bad feature because it makes it hard to see the enemies. The scorch marks from the laser cannon make that worse. Also it would be nice to have a timer until the end of the round.