Mega latias is not good in AG at all. Look at that typing, first of all. Dragon and psychic. Guess who it shares those with? The supreme Mega Rayquaza and the decent Mega Mewtwo Y. Mega Latias takes up a mega slot, but why would we use a mega slot for that when we could use a mega slot for the first AG Pokémon, Mega Rayquaza? Mega Rayquaza sports far higher offensive stats than Mega Latias, as well as a better speed. Mega latias has higher defenses, sure, but that’s not going to protect it from Yveltal and Calyrex-Shadow that are both running rampant at the moment, as well as Marshadow as you mentioned. Mega Rayquaza has Delta Stream to reduce it’s weaknesses and increase it’s resistances. Mega Rayquaza has an outstanding movepool, as well as a variety of sets including Dragon Dance, Choice Band, and even mixed sets with Draco Meteor. Mega Rayquaza is nigh impossible to reliably check, as Dusk Mane Necrozma can be broken by V-Create, Bulky Arceus forms can be broken through with boosting moves, and Zygarde as well as Giratina do not like swords dance boosted Dragon Ascents.
Moving on to Mega Mewtwo Y. Mega Mewtwo Y isn’t the best threat in the metagame, but it’s far better than Mega Latias. It sports a whopping 194 SpA, higher than any other Pokémon in the game. It also has a speedy 140 speed and a nice 120 SpD, as well as a better matchup against Fairy types than Mega Latias does. It has a large movepool to chose from, as well as boosting moves in Calm Mind and Nasty Plot, and Insomnia to help against Smeargle’s spore. It has mighty strong STAB in Psystrike, that can break past special walls such as Chansey and Blissey. However, it’s weakness to Yveltal, Calyrex, and Marshadow hinder it quite a bit, but as I mentioned, it’s still far better than Mega Latias.
Now, let’s take a closer look at some of the top tier Pokémon. We see (in no particular order) Calyrex, Yveltal, Marshadow, Fairyceus, Darkceus, Tyranitar, Zacian-Crowned, Mega Rayquaza, and Xerneas. Calyrex-Shadow sees Mega Latias as nothing but a SpA boost. Yveltal destroys it with any dark move. Marshadow can revenge kill it with shadow sneak, and can kill it easily with stronger Ghost STABs. Fairyceus and Darkceus can tank more than a few hits from it, and recover them off, as well as hitting hard with Judgement. Tyranitar takes tons and tons of special hits, and hits back with dark attacks. It can’t really do much to Zacian crowned, and Behemoth blade takes mega latias out efficiently. Mega Rayquaza takes it out if it’s holding a choice band, or has a mixed set with a Choice Scarf and Draco Meteor.
Mega Latias isn’t even on the viability rankings, and it’s not even close to them, and it never will be.
Parting words, don’t use serperior or scizor. Check out this for a general idea of the viability, though it’s a tiny bit outdated, as the DLC2 was released (spoiler alert:Caly-S goes straight to the top).