I'd personally go for this achievement once all of the other mp achievements are completed. The main reason is that after getting your 5 wins with each team, you're about half way home!
I played "RAF vs. Luftwaffe" in capture the flag mode, using the German jet plane with five stars in speed. These matches will take ~ 5 minutes a piece. Make sure that the points are set to 3 to determine the winner. It will get boring, but circle around, grabbing your opponents flag, then flying through your own flag to score. (you must physically touch both flags)
At the end of each match, hit "replay match" to avoid going through the lobby/matchmaking setup again. This will greatly speed things up as the match is instantly restarted.