The Boss Rush mode is an option on the main menu in RPA: The Ultimate Battle. In this mode, you will have to fight every boss in the game with a total of 3 characters. This can also be played in Multiplayer. This mode is unlockable and can only be unlocked by completing the Adventure Mode.
How it works
You will be taken to the Character Select Screen and must choose 3 characters to use in the mode. Once you choose those 3 characters, you can press "START" to begin or decide to replace someone else if you've changed your mind. You cannot use any of the playable Boss characters (unless you hack the game or a glitch occurs) as they will have to be fought in the mode. You will have 3 lives for each boss (1 life for each character chosen) and if you lose a life, you can no longer play as that agent until you get to the next boss. Every boss will be playable and there are five difficulties (Easy, Normal, Hard, Very Hard, Intense) with each difficulty making it harder to defeat the boss. The boss will also take less damage the higher the difficulty and more damage the lower the difficulty. The Omega X & Pixie boss fight will also be easier on the Intense difficulty due to you having less agents to choose. After you defeat all bosses, you will get a reward (customization part, trophy, sticker) and a picture for each character chosen for the mode will be unlocked.