END vs Gray fight
Battle begins with a worned less healthier Gray vs a character who was resting for most of his battle as he was unconscious when Dimaria took him out, so that means Natsu had more MP and health reserve.
On chapter 504, pg 8, Gray and Natsu get in to CQC formation, Gray gets an extra punch in during the last panel, showing he was superior in that exchange.
pg.9, both are enraged/bloodlusted, and tack their fists together.
pg.12, Gray gets a shot in on Natsu with his fist
pg.13, Gray blocks Natsu's attack
pg.14, both charge in.
Chapter 506
pg.9 Natsu charges in, with Gray in defense formation
pg.10 Natsu pushes Gray away, but Natsu detects shards of ice approaching his direction
pg.11 Natsu disperses the shards, but notices shields of ice forming at his feet
pg.12 Gray using his intellects, blows Natsu away
pg.13 Natsu uses END's roar
pg.14, Gray blocks it and uses a shield infront of Natsu's way to make him slam against the wall
pg.15. Gray uses another tactic of a block of ice to push Natsu down.
pg.16 Natsu breaks out of it. They shout each other's name
pg.17. Natsu charges at Gray
pg.18 Their fists collide. Here, Gray was clearly shown enraged, not scared whatsoever. So this bullshit of him being scared should be put to a rest, because he was right near END, and didn't even flinch
pg.19 They charge their fists equally
pg.20 Erza jumps in and blocks it.
So with all of the information given to us, it's clear that the fight overall was even, with Gray having a slight edge due to his tactics when it comes to damage applied in effect, as well as Gray having an extra punch in against Natsu from the start. So overall a healthier Gray would've defeated this END high-extreme diff.
There's no proof that Juvia healed Gray. Extra blood doesn't heal your MP or recover your overall wounds.
There's proof that Natsu rested/unconscious when Dimaria was torturing Lucy.
There's no proof that END has an extra form of being demonized, so it's rocket scientist to detect END is not partial because of lack of proof. Simple observation can get you a long way. Otherwise with that logic, Gray is only partial demonized, and not fully.
Objectively, Gray was better than Natsu in this fight, from reasons above.
Overall Gray would defeat END, and so Erza saved Natsu/END's ass in total had it been a full health battle.