Endless Fun!
This game is just simply amazing and is always nice to kill time on the job or at home when you have nothing to do. I hope there's a part 3 soon.
Also, I agree with gildedlegend's strategy, but I have a better tip:
Instead of buying the MP5K that's 300 EXP and does 1 of damage each bullet, wait a little more and buy the G36 Assault Rifle that's only 350 EXP and does 2 of damage each bullet, save up and buy the Benelli M3, max all of the powerups (Marathon Man is a must have in the beginning), and when you have enough, instead of buying the SPAS-12, buy the Pancor Jackhammer Auto Shotgun, you are practically unbeatable afterwards.