There are two pretty good options for Thornback Tortoise depending on what other bestiary entries you need.
I chose to do them in the Laguna Islands while completing Tortoise and Blood Crabs as well.
You can find the link to the map here. Also, I have made a video in this spawn if you are looking for a route to complete all these entries.
The second option is the Meriana Gargoyle Cave. This spawn has the benefit of also having Gargoyles, Stone Golems, and Tarantulas.