The Junkenstein Arcade mode in Overwatch is a seasonal event available only during the Halloween Terror weeks. It usually comes with two modes – Junkenstein’s Revenge and Junkenstein Endless, each with four difficulties – Normal, Hard, Expert and Legendary. The harder the game mode, the higher the score you get per second of survival.
The revenge mode has a maximum score as it has a maximum number of waves and the game ends at wave 12. The Endless mode, however, has unlimited number of bonus waves after wave 12. You have to try to hold the castle and survive as long as possible to get the highest possible score. If you want to challenge the highest score possible, play the Junkenstein Endless mode.