13 tiers was originally done by Lawrence Zhou. It is certainly the most optimal strat for least tiers, unmodified stats: • [BTD6] Destroying a Boss in only 13 Tiers!
Most of my strategy is mentioned in the video, but a couple of other things:
Apparently with the uboost buff, tier 5s can now be vulnerable to the "overclock bug," where overlapping overclock uptimes can cause the overclock buff to run out prematurely. Luckily Geraldo was there with his heart-shaped potions.
Much of the stalling/antistalling relied on the projectile lifespan of the super. For stalling, I put the super further away from the natural bloons (but still in range of the boss) so the projectiles would expire early. Antistalling was the opposite.
Tier 5 took upwards of 30 attempts, with a reset thrown in there for better turret spots. The statement at the end of the vid is a hypothesis based on previous runs. I'm quite surprised the boss didn't leak.
Music used:
Daniel Levi - Talk
Aeden & Sketchez - Purpose
Poylow, Misfit, MAD SNAX - Halo (I'll Be There)
Poylow & Mandrazo - Loneliness (feat. Barmuda)
[all NCS Releases]
Timestamps:
0:00 Tiers 1-4 (easy)
5:13 Tier 5 setup/initial stall
6:08 Rounds 122-130 (antistall)
7:55 Rounds 131-132 (stall)
9:24 Rounds 133-134 (pray)
10:24 Victory Screen/pops