That's not a bad set, i'm sure that in a 1vs1 scenario with Altaria going agaisnt Blazi or CharX that'd be pretty good, although regular Mega Altaria can almost freely set up DD and roost up vs those two, so wouldn't it be a waste to use that set, when the DD one (that can become a much more impactful win factor) can handle them as well?
Thanks for your reply Solerme!
I don't think a Dragon Dance moveset can freely set up on those two. Altaria that run Dragon Dance will be investing in attack and speed and therefore lack the necessary bulk to stand a chance versus powerful hits, even if they are resisted. Base stats like 75 HP and 110 Def are just average for a Mega, considering it can't hold Leftovers.
+2 252 Atk Blaziken-Mega High Jump Kick vs. 0 HP / 0 Def Altaria-Mega: 119-141 (79.3 - 94%) -- guaranteed 2HKO (Outspeeds with Speed Boost)
+1 252 Atk Tough Claws Charizard-Mega-X Flare Blitz vs. 0 HP / 0 Def Altaria-Mega: 93-109 (62 - 72.6%) -- guaranteed 2HKO (Outspeeds with Dragon Dance)
252 SpA Charizard-Mega-Y Hidden Power Ice vs. 0 HP / 0 SpD Altaria-Mega: 78-92 (52 - 61.3%) -- guaranteed 2HKO (with Flame Charge)
+1 252 SpA Volcarona Hidden Power Ice vs. 0 HP / 0 SpD Altaria-Mega: 102-122 (68 - 81.3%) -- guaranteed 2HKO (Quiver Dance)
The unboosted attacks (Return and Earthquake) from a 252+ Mega Altaria only have a very small chance to OHKO Mega Blaziken and never will be OHKOing Charizard or Volcarona. This means that should your opponent set up with one of the afformentioned Pokémon and you bring in Altaria the turn it sets up, Altaria will have to attack twice to KO them but will always be taking 2 boosted hits since its slower than the opponent, even after it uses Dragon Dance. The damage calculations show that it can't take 2 hits so Altaria would only be able to set up on them if they were brought out in battle at the very same time. Even then its matchups versus the two special attackers aren't very great.
Edit: on top of that I think Dragon Dance Altaria is a weaker version of Salamence (lower attack stat and moves with less bp), which has even more trouble with Celesteela, Ferrothorn, Venusaur, Tapu Fini, Metagross and Aegislash (to mention a few) and lacks a useful ability in Intimidate to easen setting up prior to mega evolving.