168+ SpA Mega Altaria Fire Blast vs. 252 HP / 252+ SpD Bronzong: 144-170 (42.6 - 50.2%) -- 1.2% chance to 2HKO after Stealth Rock and Leftovers recovery
4 Atk Bronzong Gyro Ball (86 BP) vs. 248 HP / 0 Def Mega Altaria: 156-186 (44.1 - 52.6%) -- guaranteed 2HKO after Stealth Rock
(since both have reliable recovery there's no clear winner here. If it's a Dragon Dance variant, Bronzong loses)
Umm, minor nitpick; but why would Bronzong lose to Dragon Dance variant of Altaria? at +1 speed gyro ball's bp increases, causing ohko on M-Altaria. M-Altaria on the other hand is not even guaranteed 2hko at +2 on a 0 def Zong.
+2 192+ Atk Pixilate Mega Altaria Return vs. 252 HP / 0 Def Bronzong: 178-210 (52.6 - 62.1%) -- 98.8% chance to 2HKO after Leftovers recovery
4 Atk Bronzong Gyro Ball (150 BP) vs. 64 HP / 0 Def Mega Altaria: 270-320 (87.9 - 104.2%) -- 25% chance to OHKO