Honestly I think you could equate the whole GS v DS situations (for all 3 encounters) to power level and stats. Zancrow was a tier above Natsu on Tenrou. Sherria was a better wizard than Wendy at the GMG. Laxus a better wizard than Orga at the GMG. I mean, Sherria with Third Origin right now doesn't stand a chance against the strongest DS (Acnologia), and she'd lose to DS wizards like God Serena, or Laxus or Natsu... because their powers/stats are higher imo.You might be right, or not. God Slayers have been shown to be more skilled than Dragon Slayers, GENERALLY. I'm talking about Zancrow and Chelia here, but not about Ogra. Apparently, Zancrow was more powerful than Natsu. Besides, Natsu could not eat his flames unless he was "empty" of his. Which leads me to think that God Slayer flames are superior to Dragon Slayer flames, since Zancrow could actually eat Natsu's flames. Similarly, Chelia was superior to Wendy. Was it due to age? Maybe. Chelia is a tad older than Wendy, and that amounts to more experience. However, Wendy has been through more thorough battles than Chelia (O6, Tenroujima, etc.). That should add up to experience and equalize her. Besides, Chelia was naturally a clumsy person. Still, Chelia was clearly superior to Wendy. Maybe she was comparable to DF Wendy. Why is that so?
I tend to think that the nature of God Slaying magic is superior than that of Dragon Slaying magic. And if that is so, it can only be because Gods are more powerful entities than Dragons. Think of Ankersham, the one of Zeref's curse. He's supposed to be a God and he's probably the most powerful entity in the FT verse, although I doubt he'll ever appear.
As for Laxus vs Ogra, it's a different thing. Not only is Laxus a "different kind of dragon slayer" (second generation I think), but also he's an exception.
The whole eating of GS flames is a good point. Could Zancrow eat Natsu's flames? I don't recall it. If that happened, then I think there is definitely some merit to GS > DS. Maybe slayers can't eat a slayer magic or something, Idk...
Yes, DF Wendy isn't strong enough. She might be an S-class, Idk if that matters because I don't think titles really matter. The level of indication DF Wendy provides is limited to DF Wendy's power level. This is someone vastly inferior to a Spriggan. How much of a chance would DF Wendy stand against God Serena? Do you think it's possible God Serena could defeat Chronos without Age Seal?Is DF Wendy just not strong enough to affect DiMaria? DF Wendy is around S-class mague, and S-class mague should be able to at least give a decent fight to a Spriggan. Wendy was being fodderized, she couldn't even scratch Dimaria, and let's remember it was not regular DF but complete DF that Wendy had. Wendy should have been able to at least "harm" Dimaria but she clearly was utterly powerless. I think this is an indicator that DF magic is not effective against gods, as much as grown-up Chelia wouldn't have been a match against a dragon but Wendy definetely was.
Could current Natsu beat DiMaria? Well, maybe. Let's remember that current Natsu is probably the most powerful mague from Ishgar. Besides, he already beat a god. Maybe he acquired some God Slayer abilities after eating Zancrow's fire, that we do not know. I don't think that's the case, though; and I think Natsu wouldn't have been able to harm Dimaria. Not because he lacks the offensive or deffensive power, or because he's not fast enough; because he's not "compatible". (Arguably, since he already beat a god)
Current Natsu would beat DiMaria (w/o Age Seal) in the same way he defeated Jacob (w/o Transport). Laxus or Serena would defeat both DiMaria and Jacob without their haxes too imo. It's a takeover, and it's a God, both types have been defeated without any slayer advantage. Essentially a wizard just has to overcome their durability and defence to cause damage. Wahl would beat Chronos without Age Seal as well I believe.
There's no doubt in my mind that having the slayer advantage or compatibility helps dramatically.But again, we're implying Dragons' phisicality here. Although I DO think it's a combination between phisicality and compatibility.
After all, Natsu defeated a god, and many FT magues defeated demons. It could very well be possible that "anyone" can "slay" a dragon, a god or a demon if powerful enough, without using magic of that specialty.
Yet I don't want to think too strictly of phisicality, since Kagura was capable of slicing through an entire town, while she couldn't even touch a dragon. What are dragons, multi-city block tanks? Kagura can cut through steel, but she cannot cut through dragon skin.
Kagura just proves that a dragons scales are much more tougher than the hardness/strength of rock, cement and other building materials. Igneel for example could level a mountain with his roar (well Motherglare's did), and that type of power doesn't damage Acnologia.
Let's just say pre-skip Mirajane and Jura were equivalent at the GMG for argument sake. It's still a very long way off Spriggan level, so to use that to gauge how well a strong human might do against a dragon is not going to work. We know dragons are really powerful. For humans, I'm just going to say Spriggan level is the bare minimum you'd need to be for comparison to a dragon like Motherglare or Zilconis. If Wahl got to fight Zilconis then I think we'd be able to make a much better comparison.I'm sorry I can't provide a source, but I somewhere read Mashima stated that if Mira and Erza engaged in a fight, Mira would win. Mira relies on skills, while Erza relies more on will. Mira is on par with Erza, which means she's apt for being considered a standard for a powerful wizzard. Mira is clearly not Laxus level, but she's Erza level. She could give some fight to a Spriggan or even beat one, if it's a weak one.
It is clear that Mira is not on a top5 list of pure human wizards, but she's probably Wizard Saint level, and if we do not consider monsters like Glidartz or Hades, she's to be feared. Unless Mashima wants to make her useless.