Excellent point Xiaoki, I would have pointed out the same for the OP. Unless the block on Sheltron somehow gets to 25% it will never be as good as TBN. Using the same example as Xiaoki, you'd have to take 125,000 dmg in 6 seconds for Sheltron to mitigate as much as TBN with 100,000 hp. The problem? Is that even if you did mitigate that much then you'd be dead anyway since it would deal your full hp worth of dmg. I don't even think that there is a way to deal that much dmg in that short of a window without killing us. Bar none, TBN is the most powerful single target mitigation that any tank has access to for the duration that is available. The problem with it just boils down to either giving it 2 more sec to more reliably pop it or grant us more MP generation to recover from it not popping.

Regarding the OP's comment on how squishy Drk is vs trash. Drk and Pld have access to the same tools for dealing with trash except for 2 things; Sheltron vs TBN and Clemency vs Abyssal Drain/Soul Eater heals. The thing is tho you really shouldn't have to use Clemency so that shouldn't really matter and as talked about above, TBN>Sheltron. If you are feeling squishy as a Drk then I could only guess that it is because you aren't using your CDs properly.

I don't have War up enough for Flash but with what I have done with it, War feels squishy AF right now. I do have Pld up enough to know I felt squishy until I got the extra 3 seconds on Sheltron. Even with that I still feel more squishy than on Drk.