If I’m being completely honest, MC-X could beat any Tobias lvl legendary 1 vs 1 with at most low difficulty and could take out at least 5 before falling (even factoring in accumulated stamina drain) after which Metagross (who pushed a Peakachu that was stronger than the variant which fought Latios) + Bisharp could finish off the remaining legendary if need be.
E4 Ace lvl is a fair estimate for Tobias’s legendaries though likely not as strong as say Flint’s Infernape. That lvl would be sufficient for feats like soloing 8 of sinnoh’s gym leaders and most league trainers (actually Agatha’s Gengar can use Darkrai Dark Void + Dream Eater strat in the form of Hypnosis + Dream Eater). That’s great and all but MC-X by the Kalos League finals is practically Champion Mega Ace lvl (at worst barely below it).
MC-X beats Malva’s Mega Houndoom after having substantial stamina drain from facing 9 other Mega Evolutions and while it healed between some battles, we know it wasn’t at full health going into the Mega Houndoom fight since we see visible bruises on it after it beat the 9th pokémon. Alain implies that Ash-Greninja’s performance exceeded that of Malva’s Mega Houndoom with his statements yet his MC-X was able to beat it despite having sustained significant damage going into the battle (as a Base Charizard against Peakachu).
To get an idea of how great MC-X’s growth rate was throughout the series, consider that in ME Act 1 MC-X had a fairly competitive fight with Remo’s Mega Garchomp with MC-X panting and having bruises after the fight. This same MC -X was able to give Siebold’s Mega Blastoise a lower mid - mid difficulty fight which shows that Mega Garchomp was no pushover either. After this until the Kalos League Remo and his Garchomp undergo intense training yet in the Kalos League rematch Mega Charizard X wrecks Mega Garchomp in 2 strikes with negligible difficulty!
Perfect Zygarde is shown to be more powerful than every trainer pokémon battling in the final battle for Kalos combined including Steven’s Mega Metagross, Diantha’s Mega Gardevoir, Alain’s MC-X, Malva’s Mega Houndoom, Ash-Greninja etc... Even if 50% Zygarde were only half as strong (and before anyone brings up the games, note that the 2 versions of Zygarde are very similar in stats barring HP which is properly doubled, so saying 50% Zygarde is only half as strong overall would be a generous low ball in this case; in addition game stats in general are objectively unreliable for the anime), that should put it well above Champion Mega Ace lvl yet Alain’s Mega Charizard X was able to A) get it’s attention with a Flamethrower as a Base Charizard, B) deal non-trivial damage with Dragon Claw as an MC-X, and C) tank both Extremespeed and more notably Zygarde’s signature supereffective Land’s Wrath after which it was ready to continue and showed no signs of being anywhere close to its limit. Obviously 50% Zygarde should be stronger but the fact that MC-X was actually putting up a fight and not being treated like fodder speaks volumes.
Don't forget he also OHKO'd Ash's surprisingly tanky Swellow, though we don't know how good Latios is defensively, as most of the damage he took was reduced by Light Screen.
Yeah that’s really not as impressive as you’d think. Swellow never displayed those tanking feats against a truly strong pokémon. For example against a wild Deoxys Swellow got wrecked by a single Extremespeed and was incapacitated for the rest of the battle (also it only landed that Quick Attack because Deoxys wasn’t paying attention towards it before it got close) whereas Sceptile later on was able to match it in travel speed and get on its back.