Play on the Sparta map.
If you're keeping an eye on when it's time to roll it'll take you and your partner just under 4 minutes each time to complete a game on this map.
After the match you stay in the same lobby and it takes just 10-15 seconds including loading time to get another match going.
This is a relatively quick boost.
If you haven't got them already legit or in local multiplayer, boosting this is also a great time to get these following achievements:
The Cocky achievement in Rock of Ages worth 24 pointsWin a match without destroying any of your adversary's units. The Eagle Eye achievement in Rock of Ages worth 24 pointsScore a direct hit on an enemy boulder with the Castlepult.