Fast method, either equip the crit accessory or use the tavern to buff your crit chance till crits are gurenteed (for base enemies its 100% boss for some reason require like 200 or something, the fire snake starts always critting at 150 ish)
Then just use the inn to buff attack dmg/ fully upgrade your weapons/level up to about lv 50 and fight it the fair way, there are multiple routes but i would probably say the fastest is max crit chance then just increase damage as after a certain point once your damage exceeds a bosses hp it can just be 1 to 3 shotted easily, cj can even 1 shot the block bar after a certain point