Although BADs normally have 55,760 RBE (98,360 when Fortified), the health of all MOAB-Class Bloons increases with each round after round 81, so the actual RBE of the BAD on round 100 is 67,200 (429,920 for Fortified BAD on Round 140)
First Strike can instantly destroy the BAD on round 100 if it has taken around 25,000 damage first (19,500 if Big Bloon Sabotage is enabled).
With Big Bloon Sabotage, the BAD reaches its last damage state (where the left and right turbines are destroyed) once it has taken 20,160 damage, which is a simple indicator that the BAD is within insta-kill range.