To first obtain this, you need to complete Son of Sparda difficulty, at which point the skill allowing Dante to use as many, or as few weapons, as he pleases will unlock in the shop for 100 Red Orbs. Start up mission 11, and use Royal Guard and Royal Revenge to your advantage, skip any encounters that don't wall you off in a red arena. When you reach Cavaliere, ensure you have some Sin DT charged (there's a white orb in the arena that'll give you a much needed boost, or sustain your RG stocks long enough to survive his onslaught of attacks). Choose any difficulty you fancy, I chose Human for convenience's sake, but you're welcome to do this on any difficulty except Heaven or Hell, as this will void the trophy.