Base Natsu at this point has better feats than DeS Gray. Even the punch he used against the Alvarez fodders is on the level of Gray's Ice-Make: Silver aoe-wise, but can actually cause damage to his targets. That's base Natsu versus Gray already half-demonized. If we add Fire Dragon King spells to the mix, then Natsu has the best punches in the series right now with demolition fist. The only spells I can think are stronger are those punches that he used against Zeref using Igneel's power and Acnologia's punches. If we factor in stuff like Dragon Force, Natsu also gets a further power boost and speed boost that outclasses Jellal's meteor. Natsu has the advantage in speed and power.
Erza was already on the same level with Kagura prior to releasing second origin. Even though Kagura can definitely make her struggle and was arguably her superior, Erza was still able to block a cut from archnemsis and hit Kagura. The difference between the two was not insurmountable. Now Erza has released second origin and unlocked nakagami armour, I see Erza as being a higher level than Kagura. She's also gone up against opponents like Ajeel, and is actually able to deal enough damage to him to knock him out.
There's also the added factor of switching opponents. I've seen nothing from Kagura to suggest she has higher durability than someone like Jura. Opponents like Bluenote can go toe to toe with Gildarts for a few exchanges, and is arguably on Jura's level. I honestly see nothing stopping Natsu from oneshotting Kagura, especially when you factor the FDK spells. On the other hand, I don't see Gray oneshotting Erza. Even if you say that Gray is stronger than Erza, she would still hold out for a while against him. She did, after all, hold out against Ajeel.