I have almost the exact same team as you, and am struggling equally with it. My original lineup was MM, Drath, Skullcrown, Sinesha, and Sepulcher Sentinel. That just got hard blocked by the valk wall. I then tried swapping out Skullcrown for Versulf - no real gain there.

Ultimately I think my main problem is just that my Skullcrown and Sinesha aren't very well geared. I think the "ideal" lineup will be MM, Drath, Skullcrown, Sinesha, Versulf, with Skullcrown and Sinesha in stun sets. That'll basically let me hard CC the valk wall.

No idea if that'll let me beat the boss though. 

Do you want to post your Knight Rev roster? And the individual builds of the champs you're using?

Well as I said, those are all my Knights Revenant that are level 60 and have all Masteries.  I have no other Legendaries but I do have most of the Epics* however they are all level 1 and sitting in my Vault.  I'm willing to work on them if one or more is essential, though.

(*I don't have Pharsalas, Sepulcher Sentinel or Bergoth)

I am more than willing to change the build of any of the Champions as needed - Msateries and gear can all change!  😀

My champs (which might not help you) to beat 21 were Wurlim, Rector Drath, MM, Doom Priest and Tomb Lord...

does the difficulty change drasticly between 20 and 21? I pretty much breezed up to stage 20 and 3 stared it and cant even get passed the first wave of stage 21 atm. 

If i regeared life steal or had a rezzer would probably be much easier but meh no rush as Im behind on a lot of other factions.

it depends on the faction. In those faction in which you face triple Valkyres at 21, I'd rate the increase as very steep. Some factions (like High elfs and Barbarians) are comparatively much easier.