Place a rocket turret on a quetz and off you go. Shoot the head as that is only way to add torpor. You could do it with canon balls but it’s harder.

Tip for single player users, if you save after getting a successful hit, kill the app and relog. You’ll avoid the wait for it to walk a certain distance before the next hit adds to torpor. This makes single player taming take an hour versus half a day.