The brotha hasnt even gone close to going all out. We likely wont get a solid explanation about his powers until final war.
Considering his (or w.e the entity empowers him) true form is this:
Using Gorosei shadows -> real size translation. He's by far the biggest enemy we'd see in terms of sheer size. It's clearly not a Lunarian/DF/Ancient Giant entity but something else.
Not to mention Joyboy didn't 1vs1 him. The mural depicts like lord knows how many vs 7. Now of course with nuance we know there's more people involved on Imu's side than 6, and same for Joyboy's side. But just the fact Joyboy's side is the one shown with overwhelming numbers and not Imu... all we need to know. Hence any proposition that Imu is getting killed in a 1vs1 is non-sense.