How to get a boss mob to spawn automatically on a block.

Yes. I'm still fairly new to MCreator however I have a bit of knowledge under my belt so I'll probably understand a slightly more complex solution. I've come up with a temporary solution where the mob spawns with the generated structure, but it isn't where I want it to be. Basically my idea is that when the structure generates, one "chained" version of the boss spawns in the center, where the player can interact with it before deciding to fight it or not. In this form, it is invincible and cannot be harmed until the actual fight is started. When the fight starts, the chained version of the boss will immediately despawn and be replaced by a "new" version of the boss, which can actually attack and be attacked. The main issue I have is that if something happens to the boss, such as it accidentally despawning due to a difficulty change or some other reason, it won't respawn again. I had originally considered just making a procedure that spawns the boss on the block, however I am not sure of how to do that without it continuously spawning the boss. TLDR: I want the structure to spawn only one instance of the boss, on one specific type of block, with the ability for it to respawn after some time.