According to the Team Options portion of the XY strategy guide for Lucario, these are good teammates for Mega Lucario:
Primal Groudon provides Stealth Rock support and can absorb Will-O-Wisp and Thunder Wave aimed at Mega Lucario while also functioning as a secondary answer to Geomancy Xerneas. Wobbuffet is an outstanding partner to Mega Lucario, as it can easily trap support Arceus formes other than Arceus-Ghost and set up Safeguard to allow Lucario to Mega Evolve and set up Swords Dance without fear of a status ailment. Thunder Darkrai can eliminate Flying-types such as Ho-Oh and Ghost-types, allowing Mega Lucario to have an easier time punishing the opposing team with Close Combat. Primal Kyogre is an especially good teammate because it can lure in Pokemon that Mega Lucario can set up Swords Dance on, such as Ferrothorn and Blissey. Calm Mind Arceus formes such as Ghost, Fairy, and Ground pressure Ho-Oh well alongside Mega Lucario; alternatively, support Arceus formes of the same types that run Stone Edge can lure Ho-Oh and eliminate it so that Mega Lucario can have an easier time sweeping. Paralysis spreaders such as Primal Kyogre, Ho-Oh, and Primal Groudon can cripple walls or Pokemon faster than Mega Lucario, making its merely above average Speed less of a problem. Any offensive Pokemon that appreciates Mega Lucario chunking at its checks can make for a good partner. Extreme Killer Arceus with Refresh applies incredible pressure to Ghost-types alongside Mega Lucario. Mega Lucario's tendency to draw in and weaken Primal Groudon and other bulky Fire-types benefits Geomancy Xerneas greatly. Healing Wish from the likes of Latias and Shaymin-S can breathe new life into Mega Lucario if it has been severely weakened or inflicted with a status ailment.
Source
Looking at this, Primal Groudon, Wobbuffet, Darkrai, Primal Kyogre, Arceus, Ho-Oh, Xerneas, Latias, and Shaymin-S are all good options as teammates for Mega Lucario.
Since you don't need a Will-O-Wisp sponge since it's a Special Lucario (I know the Smogon analysis is for a physical variant, but the suggested teammates are still viable with a Special set), Groudon is a bit hindered as an option.
Hope this helps! :)
