Same as normal Beleaguer Blitz, but the percentage is now a time limit, and now you are battling against the arena itself. To damage it, touch the cannons. You'll know it is taking damage because its HP will flash. Once the timer runs out, the project will stop itself and tell you that you've run yourself out of time. If you beat it already, click the flag twice. *NOTE: The boss has 300 more HP than you have, so be very careful about planning when to attack.*
Major credit to @Chirple for original (not bossfight) TLT (The Living Tombstone) for FNAF 4 Song Me for Bossfight Mechanics