From analysis the enemy classes which count towards this achievement are:
The eternal worshippers appear only once on each planet level (not including the very first one where it's basic robot enemies). You can mouse over the enemy and see if the tooltip says "Eternal Worshipper".
There is also usually one in the final ship level and if you see a temple that unlocks at night on a planet, definitely enter it as it's populated only by eternal worshippers.
Occasionally you'll get two together on a planet. If you do, then the console / load save exploit mentioned in the forum may be a good idea because given the scarcity of these enemies it seems unlocking will take 80 - 100 runs to get.