Hmm, this is a pretty good matchup. I would say that Erza > Mirajane, even though they're still moderately close. I think Mira gets underestimated a lot, just because Hiro doesn't like to give her good battles. Other than that Erza still outclasses her, in a high diff fight.
Now normally, I would say Gray DES is slightly stronger than Natsu in his base mode (Natsu in his partial END mode was a bit above Gray with DES. However considering only Natsu's hands transformed, and just looking feat wise, I would say that Partial END is not that much stronger than base Natsu). However Natsu has his Fire Dragon King Mode. Which puts him solidly on Laxus's tier. He alone in this mode would finish Erza, and defeat Gray. So I say Natsu and Mirajane.
Hell, a more fair matchup would be Natsu vs Gray and Erza. Then, it would be a lot closer, and maybe, just maybe, Gray and Erza would win.