The Mid Air one can be pretty easy but then I find other times, it's pretty difficult. It's strange.
Have you tried standing on somewhere high up, and shooting the enemies/creatures a bit when they are walking to you, then you shoot them more as they jump? That usually works for me. Basically get their health right down, then as soon as they jump, shoot.
In the cabin area, depending on what hive you're at, the leper usually always comes from specific directions, but some times it's one of 2 areas. From my experience, if you're at the very first (top) hive that is at the doorway to the attack helicopter, the Leper will most likely come from your right, where the other hive is, near the electric fence. Don't rush over there straight away, give it a couple of seconds otherwise you scare it off and it runs BEHIND the barn. You can still shoot it when it's round there though because you can see. If you back off a little, if he does that, he should then taunt you slightly on the little rock face on the left side near the electric fence (the bit where the meteor appears some times).
If you're in that building there plating the drill and get this challenge, he comes from the bottom at Hive 13. Pretty much as soon as it shows you "kill the leper" run down there to him, he will - or should - be in sight.
If you're at one of those other two, that lead on to hive 13 and hive 14... (this one is hard to explain but let me try)
.. as you face one of these 2 hive and plant, turn around 18 degrees. Now, on your right side at the top, is the very first hive you planted at, where that attack helicopter doorway is, right? In total you will see 3 buildings. One on the right, one in the middle, and one on the left. The middle one has a sniper rifle on the ledge. As you're looking at that middle building, enemies come down from a cliff side between that middle building and the building on your left, correct? This is where the Leper will come from. Not immediately, so don't run up there. No need to run up there at all. He will come down to you. He will literally runin a straight line, or more or less.
I've never personally had the Leper challenge when I have been at hive 13, so I am unsure where he comes from if you get it there. Has anyone had it there?
Hive 14, he usually comes from one of 2 locations. If you have your back to the hive, on the right side is a building. He should more or less come from straight ahead of you, but possibly just at your left side. You will most likely need to go run up there to get him. OR, which is more annoying, he comes up on the RIGHT side of you, where the scorpions hang out. He will run right past and jump onto the building to the right (the roof).. What you now need to do, the get him, is run ALL the way back up to the top, go right (as if you were going back to the city area). On the escape, a scorpion (or multiple) jump off that roof. This is where the Leper will come down, with about 2 seconds to go, and then he will literally stare at you, then vanish.
Sorry if my explanations suck. I wish we had access to the map. A big image that showed where the hives + building etc are. I'd show exactly where I mean for each. If anyone has a decent quality image, post it up, and I will happily do it