Her DOT stacks, even with other copies of Valeria.
Grimm, Frida, and Arthur can also help.
Blue 3*, 4*, and 5* can give lots of support to Valeria.
@Razor can you recommend any videos?
Edit
Vlad is just sad. On paper, Victor looks good. In practice only Valeria is worth ascension. Rumor from Beta, several testers proposed fixes to Vlad and Victor. Turning Vlad, and Victor, blue and giving Vlad direct damage would help a little. But swapping the stackable DOT for a health drain like Morgan Le Fay would probably help more.
Edit
Videos
(Is VALERIA worth the mats now? - #22 by Stargasm)
(https://enphero.com/Reports/Empires-and-Puzzles-2019-07-23_Reports.html?ver=3.05)