This is cumulative and will come naturally over the course of playing through the campaign, Horde and Score Attack. There are several variations of skeletons (regular, armored, armored with purple hearts), but all count towards the achievement.
In Horde, skeletons appear in wave 10. Starting with this wave, there are well over 100 if you continue playing through wave 15, so even if you are playing co-op, you are almost guaranteed to unlock this playing Horde.