Shoot for the head or tail with trans arrows and crossbow combo. When it rolls into a ball leave the area and keep an eye on them with a spy glass. Wait for it to start moving before approaching and shooting again. For high level ones, use a magnifying glass to check the torpor lvl while it's in ball form. Takes several rounds to get them knocked out. Don't waste your time trying to apply torpor while it's in a ball. Good luck.
Love these creatures! I got an ankylo as my first utility animal thinking it would get me a lot of stone, but this is guy is the way to go. I just did a run for stone on my island and got over 1,000 stone just from a few river rocks. And they were small. They aren’t bad at swimming either and I love the feature of the rolling! So cool