You can do this on a fresh save as a challenge. But if you're just looking to get the Achievement you can do this on NG+
If you beat the game on Normal and save, you can load that save as NG+ and select any difficulty you want. You will start with everything you had before, weapons, runes, items, potions, abilities, skills, etc.
If you come in fully leveled up with Bikini (Best Armor) and Sword of Verum (Best Sword) all filled with level 5 runes then you can even beat Lunatic easy on NG+ and get this Achievement.