I found that the best way to earn this achievement and the others like it are to team up with two other good buddies of yours and play team battles. All you have to do then is win!
The only hard thing about these achieves is deciding what race to do first and who gets what character! xD
In this case you will need to use the Eldar. One person will need to use the warlock (offence) another will need need to use the warp spider exarch (teleport) and another will need to use the farseer (support). Each character has various abilities tailored to their fighting style.
However, if you are new to this game and don't have steam buddies that play this game you had better make some friend fast (which shouldn't take too long). Finding friends is essential to these achieves as you will find it tough to convince two other people in the 30s before the game starts to chage their race and character!
The best way perhaps to gain friends is through TLS as you are fighting with two other people against the com so they are likely to be more friendly as they are fighting with you xD