Invisible enemies start showing up around when you enter the Prodean dimension.
The annoying part is that I believe kills only count towards this while the enemy is actually invisible. To make this easier, I've created a map with ~15 invisible enemies - You MUST play on ultra-easy and shoot a rocket at the group to kill most of them in 1 shot while invisible. If you don't get enough, finish the level by moving to the corner marked with the Ore and repeat.
The level can be found in the community workshop by searching my username or any of the words in the name of the map, "Cloak and Dagger Achievement"