Cabelas Safari

When you get up to the Leopard level, firstly equip the .375 rifle. Then go north straight away and after a short while you will see a very big hill slightly on the left, your crosshairs are probably starting to turn red at this point. Get to the top of the hill and use the sniper to look directly forward, and a little to the right, zooming in all the way. You should see a leopard far in the distance walking towards the rocks & trees to the right of the screen. This is the trophy animal you are after. I took him down then and there, by using the hunter's sense and got the "Super Long Shot" achievement at the same time.
I suggest either trying this, or moving in closer in that direction. The only problem with trying to get closer to him is that there are other leopards around that will try to attack you.