Fort Triumph

For this achievement you have to play and win as each of the four factions in Skirmish mode. The achievement doesn't care whether you win against the AI or against another player. It unlocks either way.

So, the quickest method would just be to do a 1v1 against yourself without any AI players. You don't even need a second controller, you can do each players turns with a single controller, hot-seat style.

There is no tracking on the achievement, but it popped for me after I used the above method with all 4 factions.