I tamed my first thorny by simply being on a rock ledge above it, paying close attention if any tail spikes were thrown and dodging easlie. The ledge took most of the hits and I used trank arrows to take it down. I didn't know what it ait so threw in everything. Meat, they like meat. So that's how I got my boy, Chibi.
If you are going to tame one of these with no dinos, be sure to upgrade your movement speed and get tranqs. When you see it whip its tail forwards move to the right you will dodge the shots. This worked for me.
If you have a flyer carry one to a taming pen, it's gonna aggro and start throwing spines at everything it sees the minute it lands. Plan accordingly, and use solid walls so it can't shoot through them.