Real nice one.
Can't remember how many times I play and enjoy this game.
The first weapon I bought is G36 at 350 Exp as zombies are pretty weak on the first stages, then Famas. After Famas, keep it till 3,000 Exp to buy laser cannon. Once I get a laser cannon, then the next thing is to spend 1,500 Exp on powerups "Overkill". If you got laser cannon with Overkill, you should gain 50,000 Exp in a single stage, enough Exp to buy all weapons and upgrades. Then the rest is up to you whatever weapons to use as your structure will reach 100,000 HP in no time.