Considering we're using Goodwill Yuji, I don't see a reason why Sukuna should be interfering. Sukuna as a character isn't interested in getting involved in Yuji's fights unless someone is attacking his soul, even being willing to let Yuji die at this stage. He absolutely should not be allowed to be buffing or aiding Yuji here, since he only does that in specific circumstances (like taking soul damage). Also, we've never once seen Sukuna give Yuji stat amp, the only Sukuna interferences are manifesting a mouth to talk, completely switching out with yuji or protecting from soul damage.
With regards to the actual fight, I think Greymane takes this with very high diff. They are pretty close in a lot of aspects, but the ap boost Greymane gets from worgen form makes the difference here. The fight would take a while, with stamina coming into play, and I don't see either parties going down too easily.
While it is true that Yuji consistently wins fights above his grade, the win conditions here are more than just 'kill the opponent'. Even his beating of Mahito, the one fight he convincingly won completely on his own, was super high diff and came down to that Black Flash. Since he can't choose when to activate Black Flash, I don't want to be relying on it as the tiebreaker for beating Greymane, which says to me that when going up against someone stronger than himself, he really isn't capable of killing them without random Black Flashes.
