I think Minato and Muu win high difficulty and here's why.
Minato can mark Muu with his FTG seal and then Muu goes invisible, which gives Minato a safe place to teleport to since neither Gai or Kakashi can detect Muu so they can't predict where Minato will teleport (Muu can split bodies as well giving Minato multiple places to teleport to.) This way Minato can teleport to Muu to avoid Hirudora explosion's area of effect. Kamui GG is avoided by Muu going invisible (Kakashi can't warp what he can't see) and Minato exiting Kakashi's line of sight by teleporting away with FTG. When Gai attacks one of Muu's bodies or Minato's shadow clones, Minato can teleport to the person he's attacking and counterattack him from his blind spot. Kakashi is killed by Minato warping a jinton towards him with S/T Barrier and Gai is killed by Minato attacking from his blind spot, or Muu simpy outlasts him until he reverts to base whereupon a guerilla attack kills him.