Doing head-to-head calcs for all 3 Z-Mega pairs (Serebii has Mega Lucario Z's ability named "Aura Guard", still using EVs here):
vs. 
252 Atk Absol-Mega-Z Sharpness Shadow Claw vs. 0 HP / 0 Def Aura Guard Lucario-Mega-Z: 130-153 (46.2 - 54.4%) -- 59% chance to 2HKO
252 Atk Absol-Mega-Z Close Combat vs. 0 HP / 0 Def Aura Guard Lucario-Mega-Z: 199-235 (70.8 - 83.6%) -- guaranteed 2HKO
252 Atk Absol-Mega-Z Sucker Punch vs. 0 HP / 0 Def Aura Guard Lucario-Mega-Z: 43-51 (15.3 - 18.1%) -- possible 6HKO
252 Atk Absol-Mega-Z Shadow Sneak vs. 0 HP / 0 Def Aura Guard Lucario-Mega-Z: 50-59 (17.7 - 20.9%) -- possible 5HKO
252 SpA Lucario-Mega-Z Flash Cannon vs. 0 HP / 0 SpD Absol-Mega-Z: 235-277 (86.7 - 102.2%) -- 18.8% chance to OHKO
0 Atk Lucario-Mega-Z Bullet Punch vs. 0 HP / 0 Def Absol-Mega-Z: 66-78 (24.3 - 28.7%) -- 97.9% chance to 4HKO
vs. 
252 Atk Absol-Mega-Z Sharpness Shadow Claw vs. 0 HP / 0 Def Garchomp-Mega-Z: 223-264 (62.4 - 73.9%) -- guaranteed 2HKO
252 Atk Absol-Mega-Z Sucker Punch vs. 0 HP / 0 Def Garchomp-Mega-Z: 150-177 (42 - 49.5%) -- guaranteed 3HKO
252 Atk Absol-Mega-Z Play Rough vs. 0 HP / 0 Def Garchomp-Mega-Z: 256-302 (71.7 - 84.5%) -- guaranteed 2HKO
0 SpA Garchomp-Mega-Z Draco Meteor vs. 0 HP / 0 SpD Absol-Mega-Z: 285-336 (105.1 - 123.9%) -- guaranteed OHKO
vs. 
252 SpA Lucario-Mega-Z Focus Blast vs. 0 HP / 0 SpD Garchomp-Mega-Z: 267-315 (74.7 - 88.2%) -- guaranteed 2HKO
252 SpA Lucario-Mega-Z Flash Cannon vs. 0 HP / 0 SpD Garchomp-Mega-Z: 178-211 (49.8 - 59.1%) -- 99.6% chance to 2HKO
252 SpA Lucario-Mega-Z Vacuum Wave vs. 0 HP / 0 SpD Garchomp-Mega-Z: 90-106 (25.2 - 29.6%) -- guaranteed 4HKO
252 SpA Lucario-Mega-Z Steel Beam vs. 0 HP / 0 SpD Garchomp-Mega-Z: 312-367 (87.3 - 102.8%) -- 18.8% chance to OHKO
252 SpA Garchomp-Mega-Z Earth Power vs. 0 HP / 0 SpD Lucario-Mega-Z: 280-330 (99.6 - 117.4%) -- 93.8% chance to OHKO
... It's like they want me to build around Mega Lucario Z in BSS instead of Mega Absol Z. The Mega Absol Z vs. Mega Lucario Z match-up is particularly in Lucario's favour.
252 Atk Absol-Mega-Z Sharpness Shadow Claw vs. 0 HP / 0 Def Aura Guard Lucario-Mega-Z: 130-153 (46.2 - 54.4%) -- 59% chance to 2HKO
252 Atk Absol-Mega-Z Close Combat vs. 0 HP / 0 Def Aura Guard Lucario-Mega-Z: 199-235 (70.8 - 83.6%) -- guaranteed 2HKO
252 Atk Absol-Mega-Z Sucker Punch vs. 0 HP / 0 Def Aura Guard Lucario-Mega-Z: 43-51 (15.3 - 18.1%) -- possible 6HKO
252 Atk Absol-Mega-Z Shadow Sneak vs. 0 HP / 0 Def Aura Guard Lucario-Mega-Z: 50-59 (17.7 - 20.9%) -- possible 5HKO
252 SpA Lucario-Mega-Z Flash Cannon vs. 0 HP / 0 SpD Absol-Mega-Z: 235-277 (86.7 - 102.2%) -- 18.8% chance to OHKO
0 Atk Lucario-Mega-Z Bullet Punch vs. 0 HP / 0 Def Absol-Mega-Z: 66-78 (24.3 - 28.7%) -- 97.9% chance to 4HKO
252 Atk Absol-Mega-Z Sharpness Shadow Claw vs. 0 HP / 0 Def Garchomp-Mega-Z: 223-264 (62.4 - 73.9%) -- guaranteed 2HKO
252 Atk Absol-Mega-Z Sucker Punch vs. 0 HP / 0 Def Garchomp-Mega-Z: 150-177 (42 - 49.5%) -- guaranteed 3HKO
252 Atk Absol-Mega-Z Play Rough vs. 0 HP / 0 Def Garchomp-Mega-Z: 256-302 (71.7 - 84.5%) -- guaranteed 2HKO
0 SpA Garchomp-Mega-Z Draco Meteor vs. 0 HP / 0 SpD Absol-Mega-Z: 285-336 (105.1 - 123.9%) -- guaranteed OHKO
252 SpA Lucario-Mega-Z Focus Blast vs. 0 HP / 0 SpD Garchomp-Mega-Z: 267-315 (74.7 - 88.2%) -- guaranteed 2HKO
252 SpA Lucario-Mega-Z Flash Cannon vs. 0 HP / 0 SpD Garchomp-Mega-Z: 178-211 (49.8 - 59.1%) -- 99.6% chance to 2HKO
252 SpA Lucario-Mega-Z Vacuum Wave vs. 0 HP / 0 SpD Garchomp-Mega-Z: 90-106 (25.2 - 29.6%) -- guaranteed 4HKO
252 SpA Lucario-Mega-Z Steel Beam vs. 0 HP / 0 SpD Garchomp-Mega-Z: 312-367 (87.3 - 102.8%) -- 18.8% chance to OHKO
252 SpA Garchomp-Mega-Z Earth Power vs. 0 HP / 0 SpD Lucario-Mega-Z: 280-330 (99.6 - 117.4%) -- 93.8% chance to OHKO
... It's like they want me to build around Mega Lucario Z in BSS instead of Mega Absol Z. The Mega Absol Z vs. Mega Lucario Z match-up is particularly in Lucario's favour.






