If you're gonna wear plate, you'll need a few additional things. Obsidian lute will increase mdef. Go for the v2 obsidian shield, which gives pretty much pure mdef, but provides the functionality of a shield (ie, blocking attacks). Try to get weapons that increase your spirit in particular. You'll also need mdef gems, along with a costume that provides mdef. Plate wearers can hit 6-7k without too much trouble, and when combined with auramancy skills they can get temporary boosts. In addition to mdef pots, they can get pretty high up there.
Getting mdef is much easier than getting pdef. There are no secondary stats that boost pdef, unlike mdef, which is boosted by spirit (at 1 mdef per spirit point no less).