How can I create a Pokémon with big HP, like a final boss?

[Canal_do_Lontra]

I need help everyone, I don't know in how can I create boss battle with a imense hp bar? I need help, someone can help me? If yes thanks

 

[Carboncino]

Well, you can always make an alternative form of that specific Pokémon and give it a big amount of base HP (Blissey and G-Eternatus have 255, I think you can go even higher than that). I mean, that could work I guess.
Haven't you tried Boss/Totem Battle script from Golisopod User? That could have what you're looking for, I don't know I haven't installed it cause I won't need in my project.