Right Click Any Entity Open GUI with Entity Renderer

Hello! I've seen a few ways to go at this, but I have not understood them, or they have not worked for reasons I cannot find. 
In short,
I am trying to make an undertale type battle for my mod, and I wanted to make any entity fightable. The main feature I want in this fight (the entity display renderer) is not written about that often, and I would like to have it change depending on the entity (or player) right clicked. Is this possible? and if so, would it be possible with one GUI element? And how would one do this?

Another question added on: Is it possible to make an animated fight option? like when you try and hit to middle for a critical hit, or you miss.

Thank you!