Just play with a heavy tank (diamond with two stripes logo)
The chievo will unlock after 100 matches (win or loss doesn't matter). You can back out to the garage after you've been destroyed.
You'll even get this when your tank is idle but it'll ruin the fun for your team so please play fair.
You can check how many matches you've logged at
http://worldoftanksxbox360edition.com/community/accounts/log in using your gamertag and check under Vehicle types the nr of matches played with each class.