[Scripting Question] How to Make Boss Battles like Pokemon Empyrean does?

Hi! I just wanted to know how to make some boss battles in Essentials, like they do in Empyrean, with the HP bar in the middle of the screen and like maybe multiple phases boss, like if they are mid-life, they change form or something!

Thank you so much for you help friends!

I'm refering to that actually

Well, from the graphical point of view, depends if you are using basic essentials (which version) or EBS.
If you want to make multi-stage boss, I suggest you this approach: think how Darmanitan works (Zen mode). you can copy its ability and make the boss' ability a similar one, that triggers the specified form upon Hp falls below a certain threshold.

You can also use this script