This trophy is actually fairly easy to get. Firstly, unlock TTD Location Hit Effects by getting any ranged skill to level 20. If you need to respec, you can do so on The Unreliable. It's also helpful to have the Slow The World, Quick and the Dead, and The Reaper perks to make your TTD last longer and refill faster.
Next, go to any town with a sizeable population. I used Edgewater to get mine to pop. Make sure you save before you start capping crotches if you don't want your reputation to suffer. Now, when you use your TTD and hover your aiming reticle around the groin region, you're looking for the word "weaken" to appear. That's how you know your shot is going to count.
And if you're wondering whether or not you can kill your target, you can. I was one-shotting the people in Edgewater and still got the trophy.