>>96408692
I figured out two methods:
1. Interpret Dirk Yesod's mentoring as capable of selecting past Outsiders' mentors and select Erginter's mentor Archdrakodaemon as one of them. The cyoa only says "he (Dirk) will teach you what two other Mentors you had not chosen could have thought you" which is vague enough to interpret you could pick past Outsiders' mentors.
2. Use Kashtanka of the Orthrung as a substitute mentor. Archdrakodaemon and Erginter's inborn gift stealing technique is derived from Archdrakodaemon's Yest' letter ability so if you are a Yest' letter Majin (preferably also a Majin/Drakonid hybrid like Archdrakodaemon) you should be able to learn the inborn gift stealing technique from Kashtanka as Letter of the Damned is an inborn gift so it would benefit from Kashtanka's mentoring.