I think Earthquake is the best "last slot" move for Magmortar in NU simply because you only really lose out against Miltank which a) does not tend to stay NU and b) has pretty horrible SpD so you still do a decent amount of damage to it.
Im kind of confused. Why are you bringing in Miltank? Its UU. Fighting moves hit Munchlax and Cradily specifically, but Munchy isnt that common (though Ive seen a couple today).
Grumpig has decent SDef and Thick Fat, and Magmortar only gets HP Dark/Ghost/Bug to hit it super effectively.


