>>32030551Aegislash.
252+ Atk Aegislash-Blade Sacred Sword vs. 0 HP / 4 Def Lucario-Mega: 266-314 (94.6 - 111.7%) -- 68.8% chance to OHKO
Just snek if it doesn't kill. Aegislash is basically the only non-uber legendary or Megamence thing capable of beating M-Lucario, though.