To my fellow gamers who wanted to play the game normal and are struggling with the Jurai Castle Boss i have found a simple but also long solution to beat him.
I Recommend upgrading your Attack and MP recovery speed by upgrading your character by either the sisters at sylva city by using echos(you will need to have the divine chrystal upgrade of "Nun's stations" in the fountain sylvia city) or by buying chromes and treasure that boost them. IT IS MANDETORY THAT YOU HAVE THE ADAMANTITE CHARM. you can get this at the shop at the Altomyostr training ground. With that I recommend to also carry as many Momo Fruits and Nurture-all potions as possible.
1st phase: the strategy in the first phase is to use Elicia as your "DPS" and Ran and Marie as "supports". use either Marie or Ran to feed Elicia with momo fruits to replenish her MP and keep spamming Shield Bash, this will do great damage, lower his defence and more importantly you need to inflict "confused" each turn, and with that first phase is ez breeze.
2nd Phase: DO NOT USE ANY MAGICAL ATTACKS, any attacks that require MP to use will result in an auto-fail. for this phase only use your MP for Marie's Shield, Elicia's Aegis Protection and Ran's Sacred Dance. while under protection of marie's shield you can use the plain standard attack but in this phase the adamantite charm will do all the work for you as long as you maintain shields up at all times.
3rd Phase: i dont want to spoil the 3rd phase cause it is actually very good and self explanitory compared to the other 2. My only advice is to follow the same plan as phase 2 but you can use more than jus the simple standard attack on this one.
With that you will be able to defeat the boss and finish Semen Demon sub-story.
Edit: please make sure all 3 characters have the adamantite charm equipted.