Ferro I guess, but that team already looks like something I would never use. If you only use Starmie, Milotic, and Ferrothorn you aren't weak to Magnezone. Magnezone is one of the easiest pokemon to check in the game once your steel is removed.
Magnezone doesn't seem very common anyway since it is dead weight sometimes. Also, while Mirror Coat can help a little bit against Rotom W it is very gimmicky. It is much less versatile than the other options. Scizor doesn't want to switch into a Scald while Rotom W doesn't want to switch into a Toxic.