I grinded this quickly on the Fresh Meat level. It's right after Boot Camp. Below it on the map. Near the end of the level there is a checkpoint, 2 robots and the finish line. I would fly up kill the 2 robots drop down and get killed by the lasers. Then restart from check point. Keep repeating this process 50 times. It took me about 10 mins. (7 mins on the in game clock.)