Its a buggy tame. We're lucky enough to have 2 mosas. Have your higher health mosa get grabbed using a regular saddle, platforms seem to bug him out. Have a partner feed the pearls. If it ever grabs your tame and you read it doesn't want to be tamed kill it. You can't fix the bug and you'll lose the held tame if you wait. You can always get more pearls. Your tames are irreplaceable.
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