There are so many cases when overlord or any other boss ran out from the arena they supposed to be in. So many times Yoku-ba decided to jump on somebody that stays outside, so many time overlord ran downwards the map and the list goes on and on. Simply blocking path for the bosses can fix this problem I guess.
#Make bosses smarter
steep gate
dreamy jewel
It almost feels intended when yokuba does it since it's only in last phase, punishing people standing outside