How about how it walls the musketeers and tyranitar? How could it counter them? What about other powerful sweepers like Mega Heracross (lol) and Garchomp, what about dragonite?
All right,
Keldeo:
252 SpA Life Orb Keldeo Icy Wind vs. 252 HP / 4 SpD Altaria: 148-177 (41.8 - 50%) -- guaranteed 3HKO after Leftovers recovery
0 SpA Altaria Moonblast vs. 0 HP / 4 SpD Keldeo: 168-198 (52 - 61.3%) -- guaranteed 2HKO
Virizion:
252 Atk Virizion Stone Edge vs. 252 HP / 252+ Def Altaria: 66-78 (18.6 - 22%) -- possible 6HKO after Leftovers recovery
0 SpA Altaria Moonblast vs. 4 HP / 0 SpD Virizion: 122-146 (37.6 - 45%) -- guaranteed 3HKO after Leftovers recovery
Cobalion:
252 Atk Cobalion Iron Head vs. 252 HP / 252+ Def Altaria: 158-188 (44.6 - 53.1%) -- 0.4% chance to 2HKO after Leftovers recovery
0 SpA Altaria Moonblast vs. 4 HP / 0 SpD Cobalion: 102-120 (31.4 - 37%) -- guaranteed 4HKO after Leftovers recovery
Terrakion:
252 Atk Terrakion Stone Edge vs. 252 HP / 252+ Def Altaria: 127-150 (35.8 - 42.3%) -- 93% chance to 3HKO after Leftovers recovery
0 SpA Altaria Moonblast vs. 0 HP / 4 SpD Terrakion: 168-198 (52 - 61.3%) -- guaranteed 2HKO
Mega Heracross:
0 SpA Altaria Moonblast vs. 252 HP / 4 SpD Mega Heracross: 146-174 (40.1 - 47.8%) -- guaranteed 3HKO
252+ Atk Mega Heracross Rock Blast (5 hits) vs. 252 HP / 252+ Def Altaria: 155-185 (43.7 - 52.2%) -- guaranteed 3HKO after Leftovers recovery
Garchomp:
252 Atk Garchomp Earthquake vs. 252 HP / 252+ Def Altaria: 127-150 (35.8 - 42.3%) -- 93% chance to 3HKO after Leftovers recovery
0 SpA Altaria Moonblast vs. 4 HP / 0 SpD Garchomp: 176-210 (49.1 - 58.6%) -- 98% chance to 2HKO
Mega Garchomp:
252 Atk Mega Garchomp Earthquake vs. 252 HP / 252+ Def Altaria: 154-183 (43.5 - 51.6%) -- guaranteed 3HKO after Leftovers recovery
0 SpA Altaria Moonblast vs. 4 HP / 0 SpD Mega Garchomp: 162-192 (45.2 - 53.6%) -- 32.4% chance to 2HKO
Dragonite:
This one's tricky. If I were using Dragonite, I'd use DD, take the SE hit for the WP boost, and:
+3 252+ Atk Dragonite Earthquake vs. 252 HP / 252+ Def Altaria: 236-278 (66.6 - 78.5%) -- guaranteed 2HKO after Leftovers recovery
While getting 2HKO'd by moonblast. Still a pretty solid counter
So, don't use it on Cobalion (not that much of an issue), but with a safe switch in (not that hard on Virizion or the dragons), it walls all of these.