The CB-3 cortosis battle droid, was a separatist droid unit that was almost identical to a B-2 battle droid, but was instead coated with Cortosis, making it’s armor immune to lightsabers. These droids had the exact same intelligence matrix as B-2 battle droids, but their arms were fitted with high power laser canons, and were possible to swivel, in order to block lightsaber strikes.
The only weakness in these droids was discovered by Anakin Skywalker, who found that they could only be defeated with swift overhead strikes between the arms.
Their outer shells were made out of cortosis, a substance that could block lightsaber attacks. The black outer shell was clamped to the droids’ frame in formfitting plates. The droids also had the same intelligence matrix as the similar B2 super battle droid; however, they lacked the gripping hands of the B2 battle droid, since their arms ended in double high-intensity laser cannons. The ball joint of the wrist allowed the cannons to swivel in almost any direction. The droids could also use their arms to block lightsaber blades.
There was one weakness to lightsabers in the droids’ armor, discovered by Anakin Skywalker. A very precise overhead slash with a lightsaber targeting the small gap between the two breastplates would slice a cortosis droid’s torso in half.