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.