Interesting, your 2 hand Vit DK reminded me of a build I use on my DK that I got from [1.0.6.1] Crimson Soviet: 2H Blood Knight, 4buffs/no banners Cruc. 150-170 in 10 min. (Death Knight)
but to answer about super bosses and such, the only ones I have succesfully beat 1 shot with this build has been Lokarr. I have killed secret bourbon as well, but that was a lot of deaths and just rerunning in to see it through, not a 1 shot fight, all others on ultimate have not been killable for me.
Currently my build is this with it (as you can tell its also slightly different then the build it was inspired by in the devotions)
https://www.grimtools.com/calc/w26l9RbZ
ooh and i love the cold DK idea, have been toying around with what to do for it myself.