Umm, what Mega-Tar is getting is one time power boost, and in most cases 2 out of those 5 turns is being used up for dragon dance. If Mega Tar has gotten to +2, he does not really need the sand force power boost, does he? Skarm was never going to do anything to last mon Mega Tar, sturdy skarm will whirlwind Mega Tar away nonetheless. Its overcompensating the existing issues via burst of power. Grass/Rock Tangrowth is actually worse than Grass Tangrowth, and Forry is still the setup bait it used to be. With that in mind, my vote goes to a poke who gets one of Ice Beam/Flamethrower/Earth Power and a nice stab with paralysis chance aka Mega Sceptile+Technician.