I don't know about that..No way. From my experience with it, it's way too matchup reliant to be an A+ rank mon, let alone an S-rank one. Half of the games you use it it'll be nothing but death fodder by virtue of the opponent's team being too prepared naturally for the set.
Mega latias can setup on a surprising ammount of mons.
252 SpA Life Orb Protean Greninja Ice Beam vs. +1 252 HP / 0 SpD Mega Latias: 151-182 (41.4 - 50%) -- 0.4% chance to 2HKO
252 SpA Life Orb Protean Greninja Ice Beam vs. 252 HP / 0 SpD Mega Latias: 229-273 (62.9 - 75%) -- guaranteed 2HKO
252 Atk Mega Gallade Ice Punch vs. 252 HP / 0 Def Mega Latias: 168-198 (46.1 - 54.3%) -- 59% chance to 2HKO
0 Atk Tyranitar Crunch vs. 252 HP / 0 Def Mega Latias: 192-228 (52.7 - 62.6%) -- guaranteed 2HKO after sandstorm damage
^ Well can't setup.. but that bulk tho.
252 SpA Mega Sceptile Dragon Pulse vs. 252 HP / 0 SpD Mega Latias: 142-168 (39 - 46.1%) -- guaranteed 3HKO
252+ SpA Choice Specs Pixilate Sylveon Hyper Voice vs. +2 252 HP / 0 SpD Mega Latias: 198-234 (54.3 - 64.2%) -- guaranteed 2HKO
+2 0 SpA Mega Latias Psyshock vs. 244 HP / 0 Def Sylveon: 327-385 (83.4 - 98.2%) -- 68.8% chance to OHKO after Stealth Rock
252 SpA Life Orb Protean Greninja Ice Beam vs. 252 HP / 0 SpD Mega Latias: 229-273 (62.9 - 75%) -- guaranteed 2HKO
252 Atk Mega Gallade Ice Punch vs. 252 HP / 0 Def Mega Latias: 168-198 (46.1 - 54.3%) -- 59% chance to 2HKO
0 Atk Tyranitar Crunch vs. 252 HP / 0 Def Mega Latias: 192-228 (52.7 - 62.6%) -- guaranteed 2HKO after sandstorm damage
^ Well can't setup.. but that bulk tho.
252 SpA Mega Sceptile Dragon Pulse vs. 252 HP / 0 SpD Mega Latias: 142-168 (39 - 46.1%) -- guaranteed 3HKO
252+ SpA Choice Specs Pixilate Sylveon Hyper Voice vs. +2 252 HP / 0 SpD Mega Latias: 198-234 (54.3 - 64.2%) -- guaranteed 2HKO
+2 0 SpA Mega Latias Psyshock vs. 244 HP / 0 Def Sylveon: 327-385 (83.4 - 98.2%) -- 68.8% chance to OHKO after Stealth Rock
If she has sub she can setup all over stall, if she has HP fighting bisharp and tyranitar can't be brought in, to say the least everything out there is nothing but a check depending on her set. Unaware clefable is the only nuisance to her but you should have a whole team to cover her.