The Legend of Heroes: Trails in the Sky SC - Highway Monster 15 - Thunder Quakes (Hard Mode) 

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This is actually a pretty annoying boss because the goons reflect back any Arts, so make sure you don't accidentally hit them. On top of that the bosses themselves have an attack that hits everyone, and the attack can cause Blind, so make sure someone can cast Earth Wall. Other than that, overwhelm the enemy with Earth Arts.
If you are having trouble with this game please consult this great guide: game.midoshiro....
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Eiyuu Densetsu: Sora no Kiseki SC
System: PC (Steam)
First Playthrough
Hard Mode
Playlist: • The Legend of Heroes: ...