Whoooohoooo! I finally defeated Sophie Peithos aka the final boss on my 3th try! I'm now officially done with this challange!
Even though Proto Dora took me more tries, I think it's safe to say that Sophie Peithos was this challange's toughest boss. Because against Proto Dora, I didn't waste any important items since I wanted to save them for the final boss. Against Sophie Peithos, I had to waste all my Revive DX and Rejuvenators.
Sophie Peithos was divided into 3 phases: Easy, Hard and Very Hard. It becomes "very Hard" after the first "Dark Omen". Every Dark Omen is like a package of pure pain. If you don't defend and are fully healed, you're dead... but you still can't waste turns just defending all the time. I figured out Sophie Peithos always use Dark Omen directly after using Dark Prophecy... so Dark Prophecy is your "warning signal". Once Dark Prophecy has been used, you must make sure atleast 1 character is fully healed and defending before Sophie Peithos gets its next turn.
Oh well... this was an extremely entertaining challange.
Top 5 toughest fights:
1: Sophie Peithos
2: Proto Dora
3: The 2 Gigas (that clone-gnosis)
4: Gargoyle
5: The battles on the elevator before the final boss