Not too difficult, but throughout the game, it starts adding more charms which you will be highly recommended to use against enemies (i.e. Fire Charm, Wind Charm and Ice Charm). So for that I'd advise against trying to just get the achievement by just playing the game (Though you can if you're not too bothered about waiting for this achievement, it'll probably unlock around about level 4 or 5 naturally)
The 1st level is typically the best level to attempt this achievement as the only enemies that spawn are best taken down using the Earth Charm. Doing the level on repeat should only take you 2 or 3 playthroughs to get 100 "defeats" against enemies, just make sure you're using the Earth Charm and you'll be fine. Should take about an hour or less.