The Big Merman is a Ocean sunfish. Only appearing in the Sagaia game.
This boss comes in guns a blazing.
His attacks consist of a single laser shot from its forehead, four mortar bullets released from its body in the same way "My home daddy" does (this attack can be stopped by destroying the segment of the body which releases them), wave shots from its fins (just from the one you are closer to, can be destroyed) and a ramming attack.
after the segment which drops the four mortar bullets is destroyed, he will perform his remaining attacks all at once and adding two homing missiles coming from it's head (trying to hit the head might be difficult since the missiles block your shots, and you got to evade the laser)