Congrats for getting that far! (and at such level)
Spoilers below...
Yusha is supposed to be the ultimate challenge for people who like grinding and exploring every single inch of the game, so don't worry. I balanced him around a team about lv 15 (which is the maximum level), so you're probably a little under leveled too. There's a secret gimmick to the fight that makes it much easier btw, though it's a little based on luck.
Feel free to grind some levels or try the new game+ feature to fight him later. All characters unlock their strongest skills at lv 13 so that's a good level to return to the fight too.