Well, for one, azumarill laughs at mega special lucario, and mega special lucario typically can't run priority, so it's easy to work around with scarf genesect and band azumarill.
first of all, most special lucarios carry vacuum wave, which is a STAB priority special move. Secondly, azumarill can't switch in on a lucario, as it gets 2HKO'd by a flash cannon, and only does 50% back with aqua jet
252+ Atk Choice Band Huge Power Azumarill Aqua Jet vs. 0 HP / 0 Def Mega Lucario: 133-157 (47.3 - 55.8%) -- 78.5% chance to 2HKO
after a +2 boost, it's GG for azumarill, with even aura sphere doing 60%
+2 252 SpA Adaptability Mega Lucario Aura Sphere vs. 252 HP / 4 SpD Azumarill: 221-260 (54.7 - 64.3%) -- guaranteed 2HKO