The Great Tusk is a walrus. He only appears in Darius Twin.
After promisingly facing off a barrage of sub-bosses and dismantling an Alloy Lantern who certainly looks the part of the 'final boss', you enter her maw to face the true final boss. The first thing which will strike you is that this is a singularly ugly walrus you're up against, as far as walruses go. But anyway, Great Tusk, during his battle with you, moves back and forth across the screen and can only be hurt at his head. His first attack is to fire an arc of six homing but perfectly destructible bullets, which your missiles should be able to handle. Next, he lobs a ball which sprays out bullets in a vaguely spiral fashion. Not too fast or closely-spaced, but a bit hard to see against the background. The attack only poses a real threat when he moves in about halfway across the screen, and there isn't much elbow room.
After a bit beyond the half-point of the screen, Great Tusk begins to move backwards, unleashing an arc of six little boomerangs, which don't make any particular effort to find you. As he moves backwards further, he typically fires a few more bouncing balls before starting the attack patterns again.