Trolling aside, I think Joyboy and Imu fought 1 on 1. Just like the giants, lunarians, Angels, humans helped Joyboy, Imu had help of the alliance of the 20 nations. They were most likely fighting against each other in the ancient kingdom with Imu targeting Joyboy
I'm curious about the signification behind the first 2 wars though, with probably another Nika during the first one, but it seems it's Joyboy who left the most impression on Imu