MMX is really suffering from the lack of a strong STAB. The Bulk Up + Taunt set is nice, but even at +1 he isn't OHKOing much, and Pokemon with super effective moves from the special side, such as Xerneas, Arceus Ghost, and Arceus Fairy can easily take one boosted hit and OHKO back or seriously cripple. Dragon Tail users also fare pretty good against it, such as Groudon and Lugia (Lugia can fit both Whirlwind and Dragon Tail or just use Toxic against Xerneas and Groudon just hits it hard with EQ so that another Pokemon can revenge kill, but Giratina and Gira-0 probably won't use Dragon Tail as they will be utter set up bait for Xerneas), taking little damage and phazing MMX out. Even Ho-Oh is an ok emergency check to a +1 MMX without SR up, as it can take one hit and try to burn with Sacred Fire. Or even Yveltal, which can also take a hit at +1 and OHKO back with Oblivion Wing, healing most of the life lost in the process. MMX needs a physical Psystrike.
However, a great boon of MMX that hasn't been mentioned yet (i think) is its ability to act as a great SD Mega Lucario check, one of the best there are actually. Resists Close Combat and is neutral to Crunch, Ice Punch, and Bullet Punch, while having great physical bulk and being able to outspeed and OHKO Mega Lucario with Drain Punch.