Warhammer 40,000: Dawn of War II

To get this you need to win a 3v3 game with one player as the Chaos Lord, one as the Plague Champion and one as the Chaos Sorcerer. You can use any units you like so you don't have to worry about keeping to your commanders theme.

This is really hard to boost as its nigh on impossible to be matched with another group you hope to boost with. You're unlikely to randomly get matched with players using the other two commanders; so your best bet is to get two friends and play together, with enough practice you will win a game eventually.