But an Assassin can easily tank the Fab without a swap, with my Sorc solo healing the whole fight. The shield/mitigation/self heal really helps there.
Actually, one of the more interesting aspects of that fight is that the armor debuff is 10 stacks of 16% reduction on armor. Since these modification are additive/subtractive from other armor buffs, Shadows actually end up with the most armor thanks to actually having 135% armor (which gets reduced to just 75% by the debuff). Guardians get reduced to 0% armor because the only armor buff they get is 60% extra armor from baseline Soresu form. VGs get reduced to 16% because they get the 60% that Guardians get plus an extra 16% from talents.
I think that fight is the only time in the entire game where you'll see a Shadow packing more K/E DR than a Guardian or a Shadow. Shadows are just so friggin' disgustingly amazing at it because they get to have the best of absolutely friggin' *everything* mitigation wise (K/E DR, I/E DR, self-healing, shield/abs, def). I'm not sure how the Incinerate Armor debuff on Firebrand works (I'll check out my character info next time we do it though), but, if it's just a 100% armor debuff (rather than something like a 200% debuff so that, no matter how much +armor you're packing, you're still reduced to 0 armor rating, since you can't go negative) that's been coded in the same way as all of the other armor debuffs, I expect that the same exact thing would happen: Shadows end up with more armor than the other tanks by virtue of having it reduced by less, contributing another really rare/weird/abnormal situation in which they become the undisputed kings of all things mitigation.