Crackdown 2

This achievement is very easy. I was playing with 2 other people and we all got this by accident. If you are having trouble, communicate with your teammates and have them let you attack a group of freaks by yourself. Pick up a grenade launcher or rocket launcher at the weapons drop (you can see this on your map once each round is complete) and when you hit the air lift (next to the weapon drop) it will usually warp you directly on top of the next group of enemies. Just shoot your rockets/ grenades and continue doing so and the achievement will pop after 20k. Keep in mind that a minute is longer than you think in this game, just keep firing. You get more points later in rounds and if you stay alive your multipliers can get very high. Towards the end you can get in upwards of 8,000 points for killing a heavy opponent.

Alternatively, if you have the viral achievement for playing with someone that has the WP7 achievement, you will start and have the option to use the helicopters with rockets. Just keep in mind that early on your multipliers will be low, so if you want to get this using the chopper, it might be best to wait awhile until your multiplier is higher.