This is a hilarious glitch--I have no idea if it'll ever be patched, but currently if you DROWN yourself, AND your team wins, you get this achievement! So, just find a map with deep water, park underwater, and enjoy your achievement when the match finishes!
For Tier 1, the best map would be Province, and just carefully go cliff diving in the South-East corner! (from the top of the cliff, the side approach doesn't work)
I tested this with the US T1 just now on Province, I just drove across the map, up the hill, and off the cliff. Make sure you slide off the cliff sideways though otherwise you'll probably kill yourself from the impact. LOL.Been there, done that! Achievement pops at the match results screen. If your team is terrible and loses, it might take a few tries, or you could carefully shoot some enemies first before cliff-diving!
Credit also goes to FELDGESCHREI for discovering, and MrZombieChicken and narc mouse for all helping confirm on the 360 version. And to Hanrec2 for confirming that your team MUST WIN the match.