Zenith

You can skip all the battles in the city and just rush past the knights using B. That way you can't use the merchant outside of the final boss room but you won't need to. The cooldown phase for the potions are so long that you will only be able to use 2-3 during the whole fight.

The fight itself is easy. Just attack the boss and kill the minions to trigger the next phase. The only "hard" part are the two waves of ghost soldies near the end.

I used the earth fist, red gem and earthquake scroll and this combination made enough damage to kill everything with ease.

Now comes the annoying part:
The boss fight is VERY glitchy. I had to restart at least 10 times because the MC would get stuck in an animation, a minion was invincible because its health bar was stuck somewhere on the screen or - most common - the boss would simply idle in the air without triggering the next wave. You have to restart the fight and skip all the dialogue again and again if that happens to you. VERY annoying!
Don't wait for the boss to do something. He won't move. Quit to the main menu and do it again.