Siege of Centauri

This achievement is terrible. The way I was able to do it was start mission 2 at novice. You need laser and artillery with ground fire upgrade at minimum. Rush the first three waves, upgrade colony to lv2 and put down a laser near the colony. Wait for the mobs to get to colony and start attacking.

At 5000hp left, put down two arty on the hill near where the metal foundry was to stop future waves.
At 400hp left, hit slow time.
At 160hp left, drop down a lure and reinforcements quickly.

If you time it right (and are lucky), damage should stop around 40-50hp left on colony. At that point, kill everything and the arty should take care of the rest. It took a few tries as sometimes the colony would end up around 52-60hp but at least attempts were quick.