The answer is NO. You will need to kill the 4 mini bosses only at the first time you do the event, after that you will always have permission to cross the gate that take you to the final boss.
No, its enough when you kill it once in the previous years. You can pass the gates already if you've completed it before.
Interview with Benjamin Zuckerer - managing director at CipSoftMay 07, 2020
Improving posts is now rewarded with reputation pointsApril 11, 2020