This is an easy enough achievement to obtain. Just kill 100 of the smaller Hectors in either Campaign or Survival. You must deal the killing blow.
The first stage has a small Hector at the end, and can also take out huge chunks towards the helicopter kill achievement and 1000/10000 ants achievements. Just playing through the game on all difficulties should net you almost all the Hector kills you need however. Then if you still require any, chug through the first stage a few times in order to wrap up the final kills you need for the achievement(s).
If you want to just do Hectors, stage 1 - 4 (I believe this is the right stage, the one you start off with mechs) has a bunch of Hectors and other enemies to fight.