You should be unhittable vs a lvl 82 mob (101.4% avoidance from the #s on the character window)
with raid buffs but
without shield block. As far as an unhittable set:
this is a good list of peices to help reach unhittable. I'd recommend playing around with this list and Rawr. If you use Rawr tho, be sure to set the mob lvl to 82, and if you do this, you only need to get to 100% since rawr already calculates the changes in avoidance with the lvl difference.