It's a combination of shield block/critical block/shockwave. Yes it's possible for a pally to do it, but when a warrior blocks, 60% of the time (it works EVERY time) we get a critical block, thus better. So if we are unhittable, we will take less incoming damage than a paladin. Further more with all 4 adds beating on you, each with a swing timer of .25 seconds, there is no way a pally can keep up holy shield, so it is really "passively" unhittable, not "unhittable with holy shield." This is a very crucial distinction.
When a warrior tanks the adds:
They come in, pop BV trinket (lasts 40 seconds, the adds should be dead long before it wears off), tclap then shockwave, there's 4 seconds of no damage, right as shockwave wears off hit shield block. With I think about 2000 BV (maybe a little less) shield block blocks all incoming damage, so that's another 10 seconds of no damage taken from the adds. After that warrior's unhittable/blocking skillz need to be able to keep the warrior alive for the last few seconds while dps finishes off the boss.
A lot of people will shockwave after shield block, I prefer to do it before for aggro reasons and I'm guarenteed the full 4 seconds because it is very unlikely that the pallies have holy wrathed yet to stop the shadowstrike cast.
Pallies, CAN do it, but they take more damage, don't have shield block, etc. Most people pref warriors. There's a decent argument that in this rare situation warriors are actually better AoE threat too, between the large ticks damage shield does due to really high BV, and warriors can spec/glyph glyph for a LOT of damage. I'm usually around 5k dps on that fight, 35%+ is cleave, another 25% in damage shield.
So tell your guildies, "No, I don't think I'm a Paladin, I'm better than a paladin!" /Snirk =P.
P.S. @ Pallies: hey, let us have our moment on this one! This is the first fight in a while that warriors are really really great at!!!
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