Hints:
Know what style of player you are when buying parts for your
gun. A more defensive-minded player might want a longer
barrel to take out enemies from afar, or a more aggressive
player might want a larger hopper to hold more ammo.
Hide, and hide often. On higher difficulty levels, the computer
will show no mercy in hunting you down and taking you out.
Find convenient places to crouch down and hide temporarily
until the other team has moved on... then prepare to strike.
Take notes as to where power-ups are located in each level; they
never go away, so you can go back if you're low on CO2 levels
or ammo.