Mega Miltank should become part Rock type and get an ability that's basically the same effect as holding a metronome. And get boosts to its defenses so that it can keep rolling longer
Double weakness to mach punch and to any close combat faster than its base speed won't do it any good.
Add that it is locked into a move for five turns, that it also loses its grass immunity and iron barb ferrothorn can switch in, and you realise it is a bad design for a mega.
