Easiest way to do this is with Doc. Doc's Health Shots allow him to revive himself when downed, and come with three uses.
You can even get this in the tutorial if you'd like. Select Doc and let the first enemy down you. Revive yourself, get downed again. Revive. Down. Revive. It may take a minute for the achievement to pop, mine was after I had restarted the mission.
Can also be done with the Revive Kit on yourself, but you only get one of those at a time. Using Doc's Health Shot or the Revive Kit on teammates will obviously work too, if that's more your thing.