BOSS RUSH is only available on the OMORI ROUTE during the final day in HEADSPACE. It can be accessed from HUMPHREY. OMORI and his friends can challenge simulations of the previous mandatory bosses again, such as YE OLD SPROUT.
All the characters fought there are YE OLD SPROUT, DOWNLOAD WINDOW, SPACE EX-BOYFRIEND, KING CRAWLER, SIR MAXIMUS, SIR MAXIMUS II, SIR MAXIMUS III, (Note that all 3 Sir Maximus's are fought at once) LIFE JAM GUY, SWEETHEART, MR. JAWSUM, PLUTO (EXPANDED) THE SLIME GIRLS, and all three phases of HUMPHREY.
Depending on how many battles they win, a new trophy will appear in the main room. If you lose before defeating the three SIR MAXIMUSES, a PARTICIPATION RIBBON will be displayed. After defeating the SIR MAXIMUS trio, the COOL TROPHY will be displayed. After defeating PLUTO (EXPANDED), the COOLER TROPHY will be displayed. After defeating HUMPHREY, the COOLEST TROPHY will be displayed.
Clearing the event will grant the player the "Seriously, you're the coolest!" achievement, as well as MARI'S COOKIE.