I personally love purple tanks. My Jackal +17, Poseidon +7, Drake, and Gretel raid team just eats them up…
But Ursena gives me the most trouble by far. She punishes you with an even slightly bad board and her special really puts the hurting on 4* holy heroes. Boss wolf is difficult in similar ways as Guin, but his slow mana speed makes it workable, Kunch doesn’t scare me, Hel is still A+ but isn’t MEANT to be a tank, and Aeron is serviceable but not ideal at tank.
Not viable as tank unless we are talking about a unified War Defense