Army of Two Tips

Easy Money
After you beat the game, go back to the main menu, load last checkpoint. After you kill Clyde again you will be awarded another $20,000.
You can repeat this process as many times as you want to and keep getting that $20,000. It is also possible to kill those enemies that are around Clyde with guns that have a achievements for them, SMGs, RPGs, Snipers, Handguns and so on