Let me guess. You do not use found items and upgrades on items that can increase your stats by double?
And use characters skills.
Use otsu smoke bomb/or item on him. He misses most of his skills.
And cooldown MC skills before BOSS has ~50%, kill ninjas fast and make him blind with MC/Otsu skills or item (misses most his attacks then), then uses all MC DMG skills to kill him quickly.
Goons can be wiped in 1-2 turns.