I some how doubt Nero could defeat Dante, since Dante IS the son of Sparda and we saw how Dante whooped Mundus' butt in DMC1 when he took on his Fathers form, his true Demon form.
If the Lord of the Underworld couldnt with stand the true power of Dante, how would a feeble human, even with a demon possession, a demon which would almost certainly be lesser in legend and power than Sparda stand up to Dante!
Case closed, Nero is weak in comparison to Dante, in all aspects.