They were shocked because the giant dragon suddenly started exploding magic power from all sides. If the others had been in a position to deliver the killing blow to him then yes he'd say they were strong. He was even surprised at how strong the people who defeated the other god seeds were so it goes without saying that'd he'd hold the one who kills him in high regard.
Mirajane was portrayed to rival Erza at her strongest. Mirajane fought Skullion in BSS. Nothing in the manga suggests that Mirajane actually using her full power wouldn't rival Erza using full power. If Erza was struggling or Natsu was struggling with someone in base, we wouldn't dismiss them because we know they have stronger attacks. Not that strength really affected the Skullion thing as everyone that's had trouble with him (Natsu included) has been because of his intangibility and ash hax.
I honestly don't remember you as I rarely pay attention to names on here. But my stance is that magic wise devil slaying magic is equal to fdk and transformation wise demonization is equivalent to dragon force. But forms are only one part of the equation. There's a difference in level of power used in those forms. Natsu is always going all out trying to kill his enemy when he uses DF (since mard geer). It's a last ditch effort for him so he pulls out all the stops and it's usually accompanied with Flame of emotion. Characters that use their transformations more frequently like Erza, Gray, and Wendy don't always use it to the max capability. That's why I said Natsu's feats could be replicated at their strongest. Erza can pull out new fire and ice swords but against a friend she holds back and Laxus could sense this. When she poured all of her magic into an attack, heaven's wheel could match red lightning. And even when comparing to Natsu's new secret art, Gray has recently showcased a move that's stronger than his previous moves. The destruction bow was casually stopped by Hakune but she couldn't do anything about the heavy knights. If Gray or Erza or Laxus were put into situations like Natsu vs Aldoron, we'd see them going all out with their abilities as they'd be trying to kill their opponents.
Nothing really goes against Gray and Natsu being equal to each other in series. It's just bad interpretations of Natsu's overall power and overestimations of his feats. Same with the whole "Spriggans being eclipsed" thing. People just highly overestimate every increase in power we see so they can fling around statements like "X soloes Y", or "Y oneshots X". But going by all his statements, Mashima doesn't view these increases so exponentially. And this should be immediately evident when you realize he setup the series to have multiple arcs where the main antagonists have more or less the same level of power. It's a gradual climb. Even when he setup a potential for exponential increases in power, he shut it down by having Selene be a liar and Suzaku and Natsu getting pwnd by human DG level. Although I suspect that next arc or whatever happens after that (maybe another timeskip) is going to be where we start to see exponential growth again like with the first 2 timeskips.
