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!
Won't attack you 'til you attack it. If you find some washed up on the shore take a rex or something else you have with high HP (10k HP+) and let it grapple onto it - while it's grappled jump off your mount and go in between tentacles to feed. It'll let go of your mount on its own after a while.