Don't bother wasting sweet veg cakes and risking high level dinos. Get it to grapple your mount, feed it, swim away with your mount on passive and follow, squid will let go after a while, wait above the squids aggro level until the feeding interval is up and then go back to feed it again. Rinse and repeat until tamed. You don't need to stay in the squids grasp for the whole time, affinity won't drop whilst you wait above it.
Use a Mosa to tame this beast. Let the Mosa get grabbed wait a couple seconds get off feed it black pearls. Have your mosa on follow lowest, swim a bit away from the Tuso after feeding it, when the Tuso lets go the Mosa will swim to you. Repeat until Tuso it tamed. Super easy!
You need two people, one for feeding the black pearls and the other to feed the turtle that is in the grasp of the Tusoteuthis, with sweet vegetable cake for the health and to feed it stimulates berries as well