The easiest way to get this achievement is to play a 2v2 co-op skirmish. This way you're playing against AI that are easier to beat and you will also have a teammate who will assist you.
There also appears to be a readily available supply of people playing co-op skirmishes too, so you can jump into games easily.
As far as winning the actual battle, the best bet is to spawn lots of cheap units and set them to capture outposts. This is done by pressing up on the D-pad whilst holding a vehicle (infantry seek out the nearest outpost automatically). Once you have enough of the outposts to have a good hold of the map, switch to sending your units to attack the enemy (left on the D-pad whilst holding a vehicle).
Don't forget to spend your upgrade points on yourself and get stuck in with the fight. Hopefully your teammate will be doing something similar to this and you'll win the fight in no time.