Simply start forge, then chose a wide open map such as Sand Trap or Sand Box. now get another player, it can be a friend or a conected controler. now have the other player stand in the most open space. next get a tank and move up high. next get into position so that when you let it drop that it will land on the player, killing the other Player. you should then unlock the achievement.
Note: It can be any Vehicle you want, as i got it with a Worthog.