Devastate applies a sunder.... always use devastate and dont use sunder itself. The only reason to ever use sunder is if you wanted to apply it to a crowd controlled mob. It does no damage.
There are several glyphs that will help. Glyph of cleave makes cleave hit a 3rd mob and they have a devastate glyph that will apply 2 sunders for one devastate and another that will sunder a nearby mob if you use devastate.
Glyph of blocking is good and there is a vigilance one that might be ok (I have not used that one).
Deeps for show..... tank for dough.....