In order to get your multiplier up the fastest thing to do is melee enemies. You can get the multiplier all the way up to 30 if you wish but having it over 15 is still a safe bet. Your multiplier and score can both be located in the lower right corner of the screen. The enemies in this first level are very weak. Except for a few spots, you should be able to melee everyone of them you see. After you break through the 1,000,000 point barrier, I'd suggest working on either flak grenade or vehicle form kills. The achievement will pop at the end of the level provided you exceeded a score of 1,000,000.