Defense Grid: The Awakening

You should have no difficulty unlocking this achievement throughout regular gameplay. Any foe that your towers, orbital laser blasts, or returning poison cores dispose of will count towards this achievement - you do not need 1000 aliens in one hit - it is a lifetime cumulative. So, this achievement can be earned by playing ANY level(s), except for the tutorial level. Grinder and Super Grinder missions should have over 1000 aliens each, if you want to start a game and finish with this achievement.
Best of luck!