Endzone(helper)
Another good way to get a weapon upgraded is to play endzone and try to attach a shock mod to all (or at least some) of your weapons. Since shock mods arc off enemies, you can kill a whole bunch, get lots of bolts and exprience, all while using the least amount of ammo.
We have more cheats and tips for this game here, Ratchet: Deadlocked cheats
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