Since the other solution was not that informative I figured I would make a better solution. When you choose to play arcade choose the map Subway. I found that map to be easiest. Once you gain control of your character sit back for about 20 seconds. Then make your way to the vault. By the time you get there your A.I team mates would have killed all the vault guards by then. If there is money that has not been taken by all means go ahead and take it. To get this achievement you can NOT hit any civs or team mates with your bullets. Now comes the easy part. Just sit back while you let your A.I. team mates do all the killing. Once you make it up the 2ns pair of stairs be prepared to run to the get away van. To get into the van press X when prompted to get into the van. Now all you have to do is rinse and repeat until you beat round 5. The acheivement will pop after the score screen. Hope you found this solution more informative then the other one and good luck!