(sorry about the watermarks, NVIDIA capture does not behave well on my PC for some reason)
Figure A: A screenshot of the tankbuster from the 2nd boss, moments before it snapshots and hits me. The Bole has already fallen off of me, as shown by the flying text. I have no defensives active, and the boss has no debuffs like Feint/Addle/Reprisal active (the Ewer is a Regen, the Spear is damage buff).
Figure B: A screenshot of 'literally two seconds later', after the Tankbuster 'Pummel' has hit me. It dealt 65k damage, was not Blocked (I can't, I'm a DRK) and not Parried (though the Autoattack right after it was). I was left with 65k HP after the TB AND the Autoattack, so the TB alone, with no defensives at all, did less than half of my HP. Which means that, despite Guardian now being 40% damage reduction, it would only mitigate 26k damage (out of the 141k max HP for that dungeon) from its '% reduced' effect (the part that applies to all four tanks). You know what else is 26k? The effect of Thrill of Battle, which is '20% increased Max HP' (it's actually 28k). So, we need to mitigate the enemy's damage by a factor of 40%, to match the EHP provided by a mere 20% boost to ourselves. I'd say that this illustrates quite effectively, that we're grossly overpowered compared to the enemy.
Also, I kept an eye out on the last boss of the 95 dungeon. His TB is magic, does this affect the damage dealt to me? No, of course not, it also did 65k to me out of my 141k max. At least he got to do two Tankbusters through the whole fight instead of just one.