When you first start the game, you will be automatically set to Normal Mode. Just complete all 20 mission in Normal Mode to unlock this achievement or, if you're finding it too hard for your first run, die 3-5 times to be rewarded Easy Mode. Simply change down to Easy mode, upgrade your health, devils arms, and demon trigger, then carry on where you left off on Normal using Mission Select.
Easy Mode Stats:
Lesser demons and bosses health: 90%
Lesser demons and bosses damage: 90%
Easy Mode also Features Automatic mode, which allows you to preform complex combos with a single button.
Normal Mode Stats:
Lesser demons and bosses health: 100%
Lesser demons and bosses damage: 100%