Give an advancement to a player when a mob is spawned into the world

Edit: Solved :D

-Hi, I'm making a mod where I make bosses out of my friends.

An issue I'm having is that I want to make advancements that are given to the player once the boss is spawned into the world. The problem is, the way to spawn the bosses is through structures/shrines.

I've looked on forums and youtube for days and I can't seem to find anyone with the exact same issue as me.

How exactly would I check if the entity exists in the world, and how would I give the advancement to the player once the entity exists?

I think this would be similar code to how the wither works, where its if the wither exists in the world, then all of the players in a certain range of the wither are given the advancement for it.

Thanks in advance :))