First boss of Oath was notoriously known for being hard. But the reality is when you face a boss in Ys generally (especially the 3D ones) and you notice you're not inflicting enough damage and taking too much damage, first thing you check is if you missed a piece of equipment or upgrade, as it's more equipment-dependent than level-dependent. If that doesn't work level up once or twice, but not too much. Most encounters are balanced around a certain level and it's usually the one you're expected to be at that point in the game, or very close to it.
Stat scaling is just weird in these games, one minute you'll be making no damage and taking too much, you find a way to add +5 strength and defense then *suddenly* the boss starts inflicting 1-3 damage only, while you're able to take chunks from his life bar.
Still a pretty badass series all things considered. Crept into my top 5!