Crab-II is the Gradius IV version of Shadow Dancer and the updated version of Crab. Pretty much like most Shadow Dancers, it is indestructible.
Crab-II walks around, trying to crush you. Avoid its legs and you'll be okay. However, unlike most shadow dancers except for the Ground Spider, near the end of the hallway it "fights" you in, it will begin to throw tiles of the floor and ceiling at you. The two that are the hardest to dodge are the two that it throws from it's top set of legs, while it's walking off. Avoid that, and you'll be fighting Gofer.