The boss in question is Dreadbloon: Armored Behemoth. It starts with a ceramic layer that will 'regrow' three times (each time its HP meter reaches one of the skull indicators). At higher levels it will regain some ceramic armor even between the regrowth points.

Once the ceramic layer is breached, Dreadbloon has the properties of a Lead Bloon, meaning it cannot normally be damaged by sharp projectiles, or even a Super Monkey's laser blasts (the plasma vision and Sun God upgrades should allow a Super Monkey to damage it). Use of a Monkey Village with the X/3 upgrade (Monkey Intelligence Bureau) would also allow all towers within the Village's range to damage Dreadbloon in it's lead state.

The boss in question is Dreadbloon: Armored Behemoth. It starts with a ceramic layer that will 'regrow' three times (each time its HP meter reaches one of the skull indicators). At higher levels it will regain some ceramic armor even between the regrowth points.

Once the ceramic layer is breached, Dreadbloon has the properties of a Lead Bloon, meaning it cannot normally be damaged by sharp projectiles, or even a Super Monkey's laser blasts (the plasma vision and Sun God upgrades should allow a Super Monkey to damage it). Use of a Monkey Village with the X/3 upgrade (Monkey Intelligence Bureau) would also allow all towers within the Village's range to damage Dreadbloon in it's lead state.