This can be gained really early on, during the very first mission to tame an animal.
Just after you tame the white wolf, you'll be told to use it to kill a nearby enemy. Instead you want to reload the checkpoint you just received to find yourself next to a smoking bowl between two antlers. Now walk up onto the large boulder immediately besides it and move to the edge overlooking the target area so you can just barely make out the man standing at the river's edge - you should stand far enough back that you can see the boulder you're standing on.
Here's the bit that might take some trial and error (and reloading).
You want to aim about two of his body height (think about twelve feet if that helps) above his head and launch your spear, if you did it right you'll get both the piddly 10xp for killing him and this trophy. If not, then simply reload the checkpoint and try again.
Edit: If you've already gone past this point, I'd suggest doing what I did, and look for a checkpoint during a mission right before you run into some enemies and see what you can do.