You know, if you send reinforcements regularly, you can keep the red teammate alive. This way, the orange gets caught in between you and him and it is an easier fight.
It's not meant for you to just burn through each gun.. it is designed to make you a pro or at least passable at disassembling and reassembling each gun. So stop complaining about the XP.
Seriously you should remove the credits on the gun models. Taunting us with loads of buyable stuff is one thing, but impeding progress? Why don't I just go start playing WoW, it is another one of those free-to play.. at first.