This is earned in chapter 6, verse 6. If Cereza gets hit, return to title screen then select continue to give it another shot.
Equip the accessory Gaze of Despair. It enrages all enemies and will draw them towards you - they'll actually ignore Cereza entirely. Keep to the edge of the arena as far from her as possible so wayward attacks don't accidentally scrape her and you'll have this achievement with no trouble at all.
This is also a great spot to get the achievement 'Come Here, Little Boy,' which involves killing 5 enraged enemies without taking damage. Use a torture attack on an angel with a polearm, pick up their weapon and use the B attack. You'll wipe out pretty much every angel directly around you and earn a ton of magic, ready for another torture attack.