Play the "Getting The Band Back Together" mission, and progress to where you have to bounce on ten drums without touching the ground. Instead of completing the objective, keep bouncing and killing the endless waves of OD. Use the ammunition dropped by the dead OD to keep reloading. Area of effect damage weapons such as Acid Sprinklers, Flaming Compensator, and TNTeddy are most effective. Use this trick to easily reach a 999 kill streak and get a lot of badges for weapon upgrades.