When Bellum takes Tetra, he retreats to the Ghost Ship, meaning you must follow and destroy it to get her back.

Your chief concern right now is to avoid and shoot every pink splodge that comes flying towards you. When the sky is free of pink blobs, take aim and shoot the eyes covering the ship. The top screen shows the position of all the eyes. Shoot cannonballs at each and every one to finally destroy the haunted vessel at last!!

Sailing up to the wreckage that was formerly the ghost ship, Link and Linebeck find Tetra unconscious on a part of the ship that's still afloat. Before they can reach her, Bellum snatches her up and slams into the ghost ship's mast, sending it toppling over onto Linebeck's ship and crushing Oshus who is still onboard.
There's no time to mourn his loss as Linebeck is also statched up by the slippery tentacles. Bellum quickly envelopes the captain and melds the two into an armoured enemy resembling a phantom. Lifting an enormous sword, the monster begins stalking Link!!