If you're impatient like me & don't want to one shot him on HoH difficulty, this is absolutely do-able on human difficulty. I personally played through Devil Hunter first & went back on human once Nero was pretty beefed up.
Things I found that helped;
- Staying on the far right side of the arena next to the chair, he can't hit you with fireballs.
- Attacking in the air minimises damage from a lot of his attacks if you can't avoid them in time.
- Wait to use devil trigger until his barrier breaks, you can kill him before it regenerates.
- Exceed timing your hits against the crystal help to chunk down its SIGNIFICANT health pool.
- If you die, the mission ends. So if your health is getting real low & you think you'll likely die. Pause and restart checkpoint. It's much quicker than restarting the mission completely.
Challenging, but quite a lot of fun. Probably a lot faster & easier than playing through Devil Hunter - Son of Sparda - Dante Must Die first.