This unnamed boss, most known by the nicknames "Bridge" and "Defense System", is the first boss of the Wily Battleship in Mega Man IV. It attacks with an extendable arm and the core can use special weapons from Robot Masters, varying between Ballade Cracker, Ring Boomerang, Power Stone and Napalm Bomb. It can only be damaged when the core is open. It's most vulnerable to Beat, but if the player wasn't able to obtain him, Ballade Cracker can also deal a good amount of damage.
Damage Data Chart
Mega Man IV
This boss can be compared to Main Cannon No. 2 (also known as Combo Cannon) from Kirby Super Star and Kirby Super Star Ultra. It is similar in that it is found on the bridge of the villain's battleship and the way it attacks using both an extendable arm and a cannon.