How did you decide that extra elemental damage is 25% for both?
According to this post, Ursena, Hel, Bjorn, and Mok-Arr deal +40% against Holy.
If Clarissa would deal 40% extra against Holy too, like all of them, Clarissa would deal 538 damage against Holy in your example. And that’s on DoT alone. She would also deal about 190% direct damage against Holy (135% + 40%) versus about 169% that Gravemaker deals against Nature (135% + 25%).