Managed to clear the expert version of the "To Slay a Sorceress" event, with the exception of the last 4 dungeons. That dinosaur in the 4th to last level has too much hp and hits for too much.
Found this guide:
http://ffrk.kongbakpao.com/elite-t-rex-a-zephryl-guide/
Apparently the level I got stuck at is giving lots of people trouble. The boss has 149k hp... I might give it another try with that strategy.