I've encountered two pretty challenging fights while going through the storyline thus far. The first was Elder Wyrm in Golmore Jungle, because it casted every negative status effect on all your party members every two minutes. It caught me off guard, so it got pretty annoying. The second was the ambush by the twenty-two Slimes in the Henne Mines. Holy crap, those things almost owned my party, but I stuck it through.

As for optional... the hardest I've fought is the White Mousse in the Garamsythe Waterway, because it spams Waterga before it dies, and if you cast Beserk on it, its physical attacks are pretty strong too. I also tried fighting Adremmelech, but I instantly got my butt handed to me. So Im going to wait till I'm at the higher level for that one.