How many levels are there in single player?
There are 5 levels. To maintain an 'arcade style', there are no save points, once you die you must restart. Do not worry though, once you can work out how to finish the levels it should be easier
What is the best weapon?
Especially against AI players, the mine. It is powerful and is basically needed to finish some levels. The plasma gun and machine gun are very good value weapons for doing damage in most situations. The lightning bolas can be powerful for an aggressive, accurate player (keep them stunned while ramming them).
I can't finish level 3 or 4
These levels are far easier with the mine. If you are aggressive enough to claim 2 kills in level 2, you should have enough money to get a healthy stash of mines. Just avoid the enemies and try to make them hit the mines.
How do I gain money?
There are 4 ways: surviving a level will give you $500, killing an enemy will get $1000, 'assisting' to kill an enemy (doing damage shortly before their death but not getting the last hit) gives you $100-$500, and collecting the money powerup will give you $300.
Got more questions? Send them, along with feedback / bug reports to firstname.lastname@example.org