Man, you weren't dickering when you said Dorian will be harder to beat.
My first attempt was as soon as his fight became available. It was more of accessing how strong he is than actually trying to win. Eyes were widened when I was OTK'd with two NORMAL attacks.
My 2nd was before entering Fallhault Castle while decked out with diamond gear. It went well, but I quickly realize that couldn't juggle between healing myself and attacking.
My 3rd (and currently last) attempt was the one I thought I could actually win. Just before the abandoned hospital, Lv 25, Magna Flash, all contracts completed and all perm' stat buffs I could find at the time. I was doing really well and right when I thought I was about to beat Dorian...
phase 2 happened!
Now I am really accessing if I can really beat Dorian before Chapter 5