Boring after 10 minutes
If you haven't figured it out yet, I'll tell you. It's the fastest way you I think you can ever use to get everything on the menus.
1. Buy G36 [Assault Rifle]
2. Buy Overkill [Powerups]
3. Buy Famas [Assault Rifle]
4. Buy Hollow Points [Powerups]
5. Buy Desert Eagle [Magnums].
After that, you should be able to buy just about half the list of weapons on the menu and probably all the power-ups. I didn't see any point in playing the game after you figure this out. The levels simply increase the speed of the enemies and amount, but it's nothing the Pancor Jackhammer [Auto Shotguns] can't handle...
To take up some space, I'll make comments on list of weapons...
[Pistols]
USP: Crap
CZ 97b: Crap
Five-seven: Crap
[Magnums]
S&W .357: Crap
Taurus .454: Crap
Desert Eagle: This + Overkill upgrade = Godly exp maker.
[Sub Machine Guns]
MP5K: Never need it
P90: Never need it
[Assault Rifles]
G36: Great for starters... until you get the Famas
Famas: Better version of G36. Best friend for a few rounds
FN SCAR: Great for pushing back enemies. Beware of reload time
[Shotguns]
Mossberg 500: You want to die?
Benelli M3: You're dead
SPAS 12: Great weapon for dishing out mass kills
[Auto Shotguns]
Street Sweeper: Reload time = death
USAS 12: Crap
Pancor Jackhammer: BEST DEFENSE TOOL EVER
[Light Machine Guns]
M-60: Crap
HK 21: Mow down enemies in lines
[Miniguns]
M-61 Vulcan: Reload time = death
GAU-8: Reload time will kill you.
[Laser Cannons]
XM 58 Laser Cannon: Crap. Cool effects. Shoot once and never again.
I've played about five moer minutes to see if anything different happens, but I am disappointed to say, they just get faster and "stronger."
Please copy + paste the strategy if you want to move on in your life.