Yun should try assassin/sneak build, would improve his effectiveness
Yun is half way there already. He has everything needed for a stealth archer other then actual stealth. But her unbicorn kind of covers for that.
But Yun's build is backline support, and he excels at that. Going to stealth or assassin would not actually improve his perfomrmance in a party. Though this story is in a game where you can swap builds easily and Yun kinds should kind of have picked up stealth for when he is going solo.
One of the points of the story is that the standard hyper focused murder hobo only good at 1 thing builds (like pretty much evert combat focused beta tester we see has) are actually garbadge in the game, and it is the synergiesa from real life (Job and monster killing) builds like Yun's are where the strengtrh is. I mean every "hard core" player we meet spends much more time leveling then Yun, many also spending more time in the game, but Yun's combat secondary build still holds up in a fight every time he gets in a party, they should be leagues ahead every time but are not.
The thing is Yun could be a lot strogner with just the senses already he has, but his focus is not direct combat.