Emerald so far as an optional Boss, but the 4 Sealed Monsters in FFV in X-Death's forrest was a pain in the ass. I faced him literally 50 times and took a 4 month break from the game just because I was so sick of doing the pre-boss battle routine due to the fact I was basically re-enacting the route in my dreams.
I eventually beat the 4 Seals then when I figured out how good the Samurai was...damnit.