Ratchet & Clank: Up Your Arsenal Tips

1 weapon challenges at annihilation nation
When you have the 1 weapon challenges, do one of the following:

#1: When your in an enemies line of fire, move in front of another enemy so that the enemy you want the other guy to hit is in front of him. It will kill more of them off, conserving precious ammo.

#2: Before you enter that challenge, train with that weapon and get it more levels up. The more level ups it gets, the more ammo it has.