Hover Skiff makes this easier but isn’t necessary! They aggro very easily to player structures, I got my first one mad at a stone dino gate on the ground. Once it’s stuck to the gate, put a couple cannons down (this is important- they’ll run away faster than 1 cannon in most cases) and put em to sleep. 2-3 cannons and 3-4 cannonballs.
Singleplayer taming is so hard. I had nobody to aim the cannon, and I'm currently waiting for the astercetus to wake up. I struggled so hard. He's a blue level 8. LEVEL 8. Dododex says there's a 93% chance of death for the cannon for a level 8.. Pretty lucky, eh? Singleplayer taming is.. Hard. Not impossible, though. I managed!