To unlock O´Sole Meeo in the character selection you need to kill 3000 Dragon Shrimps (the enemies that kinda looks like Onix). They first appear on Gallo Tower (Stage 4).
There are different types of Dragon Shrimps, but as far as I can tell, it doesn't matter which type you kill.
Once you killed 3000 Dragon Shrimps you can then unlock O´Sole Meeo in the character selection for 500 gold.
For me, I only needed to reach lv 80 with O´Sole Meeo (in the Unlocks menu on the start screen, it says to reach lv 80 with O´Sole Meeo). Once you have completed the criteria for the Unlocks you should also get the achievement. The lv 100 criteria is from an older version of the game (which I guessed changed for the final release of the game).
Happy Surviving!