I tried the healing thing but it didn't work as she would one shot Eileen and tie her same turn. I think this game expects you to be a certain level but if you haven't been fighting much enemies you can go in hideously underleveled. Also the bosses attacks are random so she can spam oneshotting attacks one attempt then never use it on another.
Viewing post in Didnapper 2 | v1.2.0 Truth expansion update comments
Oh yeah, I was horribly underleveled for this fight. Plus, I never noticed until afterwards but I had a ton of agility boosting cards I hadn't used yet and would've helped me address the number of turns she was getting before I could react. I did get lucky on this run and was able to just about keep Seres up while Eileen and the other party members were struggling free. Took me a long, tense time, but I did manage to just barely beat her. Other than the occasional beef gate of a boss (to be fair, this has been the only one so far) I've been really enjoying the expansion. Amazing that we got so much new content for free!