When you beat him - my advice: don't try to fight the swarm. It's possible, but hard, and you can die easily. You will know when you'll find them.
it would be wonderful to have the soundtrack in the extras. Great game!
1. Enter the next area, get the double jump, but don't fight the boss yet. (You might want to kill enemies to get experience until the enemies don't give that much.)
2. Head to the mountans (you're not supposed to go there yet, but you can with double jump). Explore that area, killing the monsters. (If you didn't level up in step 1, you might have serious difficulty and may need to save after each enemy at first.)
3. Go past the save point, past the cabin (and past a boss that doesn't spawn yet), and into the later part of the area. You will find stronger enemies to kill that in turn will allow you to level up even higher. You will eventually reach a point beyond which you can't continue.
4. Spend the money you obtained on fourth tier equipment.
5. Now, go back and play through the game as intended. The next two boss fights should be ridiculously easy, even if you are playing on Nightmare. The following boss, however, will still be a nightmare (is there a good place to level up for that boss fight?)
It may be possible to break part of Ys 6 in this way (if you can kill any of the slimes in that one cave), but not Ys Origin, which is more linear.