Perang Cemen

Tbh(to be honest) I got this idea when I was playing ff6, I was defeated by Magic Master in top  of kefka tower.

The damn thing is when I manage to defeat Magic Master, he cast Ultima that deal 9999 damage to all allies.

I try 10 times until I manage to defeat him without got killed as well.

Since in my game there no kind of Life 3 spell or reraise or something... I make the boss cast it when their Hp drop 50% below,

Oh right that skill has 10 priority while other are 1 so after the boss Hp drop below 50% they will cast it in their next turn or asap.

Gfufufu... I wonder how many times player defeated by my boss until they realize how to defeat it.

Or maybe they will stuck? who know, the game is still wip (work in progress) anyway...

This is the kind of skill that would make me instantly stop playing your game, even if there is a hidden 'work around', thanks for letting me know in advance so I don't waste the time.

Something like this could never be enjoyable for a player, you may re-consider implementing it.

Be very careful with this sort of design.

Just because you have some sneaky work around doesn't mean the player will pick up on it, even if it's blatantly told to them. If it's told in a cryptic way, the player might not find the right answer. Even if it's very obvious, you can't expect players to remember every hint they come across. Say, they put the game down for the night. By the next day, they'll have likely have forgotten some of the details. And worse yet, if it's something you don't even hint at, and expect them to trial and error. 

With that said, there are ways you can do this Ultima attack, and still have it be both fun and fair. 

Also, this doesn't directly apply here, but be careful about taking inspiration from FF. They may be the poster series of RPG's, and have done a lot of things right, but they're far from perfect.