Dual Crater is the first boss of sector D. A dual hulled assault craft/aircraft carrier hybrid, the mission is to destroy it before it reaches the beach and deploys it's troops, effectively putting a time limit on the player who must defeat it before then.
Dual Crater itself is not an effective fighter. It is only lightly armed and will move in a straight line towards the beach. It isn't especially fast either. The strength of it comes from the swarms of fighters and PT boats it will launch. The PT boats have 4 (yes 4) hit points, so a hit from almost anything will sink them immediately, but they will launch devastating swarms of torpedos in the numbers they spawn in and are tiny, so taking them out is a must. The best way to counter both these and the airplanes is by using ordinary or new flamethrower guns, which will wreck the PT boats in large groups and damage aircraft as they try to take off or land. Dual Crater has twin weakspots at the back as when the PT boats launch it leaves the inside of the boat exposed. Firing into these weakspots will deal far greater damage, so aim your heaviest guns into the openings.
There is no second boss on hardmode after this fight.