Altaria-Mega @ Altarianite  
Ability: Pixilate  
EVs: 252 HP / 56 Atk / 24 Def / 176 Spe  
Jolly Nature  
- Dragon Dance  
- Return 
- Roost / Heal Bell
- Earthquake / Fire Blast

Role
Altaria Mega functions as a bulky physical setup sweeper and can function as a cleric, depending on the set you use. Its advised to send it out when you can at least set up one dragon dance. 

Set
56 attack and 176 speed EVs with a jolly nature makes it possible to outspeed and 1hko tapu koko and nidoking with earthquake at +1. The 24 defense EVs are there to deal with Groudon-Primal better, because precipice blades does around 50% damage with a physical offensive EV spread on Groudon-Primal. If you want more EVs in defense and nothing in attack, then its best to make sure that there are rocks on the field, making it still a 1hko against a bunch of mons after rocks (see calcs). You can also run 228 Speed EVs instead of 176 to outspeed Timid Mega-Mewtwo-Y after a dragon dance. 

Calcs (Offensive)
+1 56 Atk Pixilate Altaria-Mega Return vs. 0 HP / 0 Def Mewtwo-Mega-Y: 301-355 (85.2 - 100.5%) -- 87.5% chance to OHKO after Stealth Rock
+1 56 Atk Altaria-Mega Earthquake vs. 0 HP / 4 Def Tapu Koko: 282-332 (100.3 - 118.1%) -- guaranteed OHKO
+1 56 Atk Pixilate Altaria-Mega Return vs. 0 HP / 0 Def Tyranitar-Mega: 318-374 (93.2 - 109.6%) -- guaranteed OHKO after Stealth Rock
+1 56 Atk Altaria-Mega Earthquake vs. 212 HP / 0 Def Marowak-Alola: 226-268 (71.9 - 85.3%) -- 75% chance to OHKO after Stealth Rock
+1 56 Atk Altaria-Mega Earthquake vs. 0 HP / 0 Def Groudon-Primal: 164-194 (48 - 56.8%) -- guaranteed 2HKO after Stealth Rock
+1 56 Atk Altaria-Mega Earthquake vs. 0 HP / 0 Def Nidoking: 306-362 (100.9 - 119.4%) -- guaranteed OHKO
+1 56 Atk Altaria-Mega Earthquake vs. 4 HP / 0 Def Excadrill: 374-440 (103.3 - 121.5%) -- guaranteed OHKO

Calcs (Defensive)
252 SpA Choice Specs Tapu Koko Dazzling Gleam vs. 252 HP / 0 SpD Altaria-Mega: 422-498 (83.7 - 98.8%) -- guaranteed 2HKO (inherited 154 SpA)
252 Atk Groudon-Primal Precipice Blades vs. 252 HP / 24 Def Altaria-Mega: 226-267 (44.8 - 52.9%) -- 27.3% chance to 2HKO
252 SpA Choice Specs Magnezone Flash Cannon vs. 252 HP / 0 SpD Altaria-Mega: 374-444 (74.2 - 88%) -- guaranteed 2HKO
+1 252 Atk Tyranitar Stone Edge vs. 252 HP / 24 Def Altaria-Mega: 226-267 (44.8 - 52.9%) -- 85.2% chance to 2HKO after sandstorm damage
252 SpA Mewtwo-Mega-Y Ice Beam vs. 252 HP / 0 SpD Altaria-Mega: 250-296 (49.6 - 58.7%) -- 99.6% chance to 2HKO
252+ Atk Thick Club Marowak-Alola Shadow Bone vs. 252 HP / 24 Def Altaria-Mega: 259-306 (51.3 - 60.7%) -- guaranteed 2HKO (inherited 120 Attack)