You know, you don't have to resort to templates...
Make the momma rust monster the result of a botched Polymorph Any Object spell where the wizard is now stuck in that form. You then have a highly intelligent (and possibly Eeeeeevil) rust monster on your hands. The party won't expect the familiar of the BBEG to belong to the rust monster, nor will they know what to do when Silenced and Stilled metamagiced spells start flying at them... from the Rust Monster. And the fighters can't do their usual thing and charge the wizard... because the wizard/RM will rust away their stuff.
-or-
16th level polymorphed Fighter.
16/11/6/1 antennae attack progression.
16d10+Con for HP.
9 Fighter feats.
Int, Wis, Cha that allow for tactics other than "charge and eat metal!"
Make the momma rust monster the result of a botched Polymorph Any Object spell where the wizard is now stuck in that form. You then have a highly intelligent (and possibly Eeeeeevil) rust monster on your hands. The party won't expect the familiar of the BBEG to belong to the rust monster, nor will they know what to do when Silenced and Stilled metamagiced spells start flying at them... from the Rust Monster. And the fighters can't do their usual thing and charge the wizard... because the wizard/RM will rust away their stuff.
-or-
16th level polymorphed Fighter.
16/11/6/1 antennae attack progression.
16d10+Con for HP.
9 Fighter feats.
Int, Wis, Cha that allow for tactics other than "charge and eat metal!"