The Ancient Battleground was a pretty good fight too (at least when I got there). And The Engineer if you choose to be a tight-wad with him.
Josaphine is certainly the most powerful single creature. John the Dragonrider is probably the next toughest.
Personally, I enjoyed the larger battles a lot more than the bosses. That's probably because I played as a full-melee warrior. It involved a lot more of deciding who to go for first and when I need to run for recuperation. Versus a boss, it's just click and then ready on the food/health potions.