This is a pretty easy achievement that can be done in online coop on any difficulty.
Basically in the mission unconfirmed you will be instructed to go into some mines, and after a small firefight you should see a gungoose at the bottom of some stairs. just get in the gungoose with your buddy, and simply kill 30 enemies. the kills are combined between you and your partner, so both of you will get the achievement at the same time. Enjoy your 10G