This achievement, as frustrating as it can be is actually quite simple, mow down one hundred enemies with your heavy weapons.

The trick is to stick to the enemies on your own track (except the snipers).
Shoot every enemy you can as soon as they appear on the first two waves,
if you let them get too close the other AI will kill them.

Don't even bother lifting the sniper rifle, the animations take up a lot of time.
Keep shooting with the Gatling gun and learn the positions and timing of the snipers, you'll get them no bother. Spray and pray!

After the Shields / sniper run you'll be offered a grenade launcher, don't take it just yet.
Keep mowing down the snipers that continue to appear. This is where I got my achievement.

This achievement can be completed on any difficulty but I'm not sure if dying voids it or not.
Maybe someone can confirm if they try it?

Best of luck, I hope this helps.
FM