Aberrations are very common, so you should unlock this in one or two playthroughs. Note that Foulspawn are a subtype of aberrations, so if this achievement is left over, you should go kill foulspawn in the chasm, through the teleport to apprentice row. That way you will earn progress towards both this achievement and
Note that in the in-game tracker only counts kills per character, not overall. Use the Achievement app tracker instead.