This achievement is possibly one of the easiest in the game. All you have to do is win a Skirmish game with each of the leaders, as the description suggests.
The easiest way to do this is to play 6 1v1 matches, using a different leader every time. Set the A.I. to EASY, (doesn't matter which leader they are), and make sure that the mode is DEATHMATCH. This will make the game as quick as possible, so as to take you the least possible time. (The bits in capitals are just so that they stand out - I'm not shouting at you)!
The best map to use is Blood Gulch, owing to the fact that it is so small. HOWEVER, (I know, I'm doing the 'capitals' thing again), if you change the map each time, you are a little bit nearer to getting the "Gallivant Around the Galaxy" achievement, which is win on every single map.
I hope this helps all of you who were wondering about "strategies" for this achievement.
Happy Hunting people!