This phantasmal creature can be slain just like the rest.
Get into a well spaced octagon with some VERY trustworthy individuals. If everyone turns their back at the same time at equal distance, the sharks will amalgamate at the center. They will all split apart when they try to eat you, killing the creature(s) once and for all.
This is the only way and DO NOT TRY WITH OTHER SHAPES DO NOT ADD DIMENSIONS TO THE RITUAL