Zenon should take this Mid-high difficulty.
I can't gauge post-ts Mereoleona as of yet so I base this on the information we have up to the Mereoleona's final appearance.
I give it a Mid-high difficulty because Mereoleona is ridiculously fast and I assume that Caldius Brachium should be a nice counter to Zenon's Bone magic. Although as I say time and time again. There is no way Dark Triad member loses to one Captain level character.
It's just impossible.