Based on what? With its type adventage, it should've beat Ash-Greninja more than once to even be close to E4 level
Mega Sceptile is E4 ace level whereas Ash-Greninja was E4 Mega Ace level. The gap between them was so big that a mere type advantage wouldn't be enough. I think you made the mistake of thinking that Ash-Greninja was equal to Mega Sceptile, when actually Ash-Greninja was basically twice as strong as Mega Sceptile. If Alain's Mega Charizard X as it was in the Kalos League battled Siebold's base Blastoise, then Blastoise would get dominated by MCX just as hard as Ash-Greninja dominated Mega Sceptile.
I think it was Clemont (or some other character in XY) who mentioned the boost from Mega Evolution doubles a Pokemon's overall ability, in other words the boost from Mega Evolution moves a Pokemon up one tier. Bond Phenomenon is even stronger than Mega Evolution, boosting Greninja up two whole tiers (hence why Ash-Greninja can keep up with MCX). So Ash-Greninja vs Mega Sceptile being in the former's favour is what literally anyone should have expected.
To make a DBZ analogy, Ash-Greninja would be like having Super Saiyan 1 Vegeta fight Super Saiyan 2 Goku with both fighting at full power. The only way Vegeta could win in this case is if he were twice as strong as base Goku; the only way Sawyer's Mega Sceptile could beat Ash-Greninja is if base Sceptile were an entire tier above base Greninja (which isn't true, since both are equal in base).
Because AG didn't have to hype up Ash's rivals to make him look strong without letting him win the League (*cough* Tobias, *cough* Alain)
So we're talking about the same league where Ash vs Katie (literally a CotD) was unironically better than Ash vs Morrison or Ash vs Tyson. AG didn't hype up Ash's rivals because he basically didn't have any main rivals in AG; Morrison was basically a League rival with the way he was handled.
That isn't something to praise AG for; Ash not having a main rival in AG was simply bad writing. The only reason AG didn't have a Tobias/Alain type character is because Ash didn't have any main rival whatsoever.
Imagine if DP never had Paul whatsoever and Ash's only rival in that series would be Barry who doesn't debut until after Ash already beat Volkner, with the full battles literally Ash vs Random CotD, Ash vs Barry, and Ash vs Tobias... that is basically what the Hoenn League actually was.
I mean it's quite clear XY (and to a lesser extent DP) rivals seem so powerful because these series needed it to justify Ash's League loss - one strong trainer for Ash to beat (Paul/Sawyer) and one Gary Stu to beat Ash (Tobias/Alain). Had they appear in OS-AG, they wouldn't be as strong.
No, DP/XY rivals were powerful because they were powerful. You're overthinking things. If the writers wanted to justify Ash's loss in XY, they never would have given him Ash-Greninja. They could have just not given him Ash-Greninja, nor given Sawyer and Alain any Mega Evolutions. That way, Alain's base Charizard would still beat base Greninja 1v1 in the Kalos League finals. Why? Because Alain's base Charizard is E4 ace level. Base Greninja and Sawyer's base Sceptile are stronger than BF Pikachu or Brandon's Regice/Registeel, but probably weaker than some of the other Pokemon in the same tier (i.e. Paul's Electivire, Infernape, BF Sceptile, BF Charizard).
Also, the burden of proof is on you. You say that if they appeared in OS-AG that they wouldn't be as strong, but you can't prove that. Harrison was a stronger trainer than Tyson, despite AG taking place after OS. Cameron appeared in BW, but I don't think anyone in their right mind thinks that Cameron would beat someone like Tyson or Harrison. If the DP/XY rivals appeared in OS-AG, they would have been just as strong as they would without Mega Evolution.
If Paul/Sawyer/Tobias/Alain appeared in OS-AG, they would still be just as strong as they were without Mega Evolution. Less so for Tobias, but he would still be equally as strong in OS-AG. In AG, he could use Latios, in Johto he could still use his Latios since Harrison had two gen 3 Pokemon on his team, and Tobias could just use Entei or one of the three Kanto birds if they had him in the Kanto League. Alain would still be just as strong since at least half his team is on-par with Tyson/Harrison's regulars while Bisharp is a fair bit stronger than Tyson's Metagross. The only notable Pokemon that Alain had were base Charizard (E4 ace level) and his Metagross (stronger than BF Charizard, but weaker than the E4 ace level BW Charizard).
If we ignore Sawyer's Sceptile and Tyson's Meowth, both their teams are overall comparable. So Sawyer would still be just as strong in OS-AG as he was in XY without Mega Evolution. Paul would also still be just as strong since he could have used any of his Pokemon. 4 of his 6 Pokemon from the Sinnoh League team would be around the level of Tyson/Harrison's regular Pokemon. In AG, simply swapping Electivire+Drapion for Ursaring+Magmorar would put him as a reasonable League crusher for Ash. In Johto, he could literally use his Sinnoh League team without any changes. In Kanto, we don't know how strong the very strongest of the Kanto League were when Ash entered it, but based on how it was when Ash entered, Paul could probably win with the team he had when he battled Cynthia (Torterra, Elekid, Murkrow, Weavile, Chimchar, Ursaring).
TL;DR: Ash-Greninja was much stronger than Mega Sceptile that the latter's type advantage was completely irrelevant. AG had terrible writing for the Hoenn League. DP/XY rivals seemed so powerful because they really were that powerful. If they were in OS-AG or in BW, they would still be just as strong as they were without Mega Evolution, especially in the case of Paul and Sawyer.
