Altaria calcs

252+ Atk Primal Groudon Rock Slide vs. 0 HP / 0 Def Mega Altaria: 41-49 (27.3 - 32.6%) -- guaranteed 4HKO
252+ Atk Primal Groudon Fire Punch vs. 0 HP / 0 Def Altaria: 49-58 (32.6 - 38.6%) -- 99.4% chance to 3HKO
252+ Atk Primal Groudon Fire Punch vs. 0 HP / 0 Def Mega Altaria in Harsh Sunshine: 63-74 (42 - 49.3%) -- guaranteed 3HKO
252 Atk Life Orb Mega Rayquaza Dragon Ascent vs. 124 HP / 88 Def Mega Altaria: 144-172 (86.7 - 103.6%) -- 18.8% chance to OHKO
Higest HP + 36 speed + 4 SpD + 8 Remaining.

Loses to +1 or Helping Hand boosted Ray reguardless.

252+ Atk Parental Bond Mega Kangaskhan Double-Edge vs. 132 HP / 88 Def Mega Altaria: 139-166 (83.2 - 99.4%) -- guaranteed 2HKO
Add 8 HP makes it live Double Edge 100% from Adamant M-Kanga.

However!
252+ Atk Parental Bond Mega Kangaskhan Double-Edge vs. 132 HP / 88 Def Mega Altaria: 141-168 (84.4 - 100.5%) -- 0.4% chance to OHKO
Using the VGC 16 Nugget Bridge calc, still the 8HP makes a slight difference even in that unlike defence so.... I will continue to use that.

248+ SpA Altaria Hidden Power Water vs. 0 HP / 4 SpD Primal Groudon: 112-132 (64 - 75.4%) -- guaranteed 2HKO
248+ SpA Altaria Hidden Power Water vs. 252 HP / 4 SpD Primal Groudon: 112-132 (54.1 - 63.7%) -- guaranteed 2HKO
248+ SpA Pixilate Mega Altaria Hyper Voice boosted by Fairy Aura vs. 4 HP / 0 SpD Mega Kangaskhan: 97-115 (53.5 - 63.5%) -- guaranteed 2HKO
248+ SpA Pixilate Mega Altaria Hyper Voice vs. 0 HP / 4 SpD Yveltal: 198-234 (98.5 - 116.4%) -- 81.3% chance to OHKO