i've had this happen once, when in persuit of it, it fled over a house.
and whilst the timer had not run out, it showed challenge failed.
you're lucky it actually comes after someone. on one occasion, it went hiding behind the houses in the city, next to the hive blockade.
absolutely no way of killing it, so had no other choice then let the challenge fail.
I usually drop a booster so I can detect it from far away, move in and unload on it.
I do think that from all the challenges this one is the worst. even 50% acc. in the cabin is better.
The Leper is really easy to find once you know what you are looking for. There are several ways to spot him and it's always in the same locations.
Ways to spot him:
1. If you're in the first area with the hotel, he is the biggest alien on the map, he is gray and black (doesn't glow red like normal).
2. If you are in the city/farm, He looks like a hunter but he has no red on him, he is just gray and black.
3. HE WILL NEVER ATTACK YOU, so look for the black and grey hunter that is hauling butt past you.
In the first area, he always comes down the hill by the sniper and marksman rifles toward the electric fence and motel sign.
In the city, he always comes down off the two buildings by the barrier to the farm. (he runs to the right, jumps the fence and gets away if you take too long to kill him).
In the farm, he always comes up the middle of the map from the hive at the very bottom (right side of the nuke hive), he runs toward the barn so stand in the middle of the farm and keeps you eyes peeled.
With this information you will kill him 9/10 times instead of failing the challenge.
Note: This is my results from almost 200 games, The only time I ever fail this challenge is if I am running all five relics and have no ammo in my pistol. I have over 50 escapes and I do have the completionist achievement.
"In the city, he always comes down off the two buildings by the barrier to the farm. (he runs to the right, jumps the fence and gets away if you take too long to kill him)."
I thought this, because I have seen it before. However, earlier we had it in this area, and we all went rushing to that area near the traps, and the leper definitely did not come by. The timer went all the way down to 0. So, we didn't even get the chance to kill it
This seriously needs fixing. A challenge surely must be doable, otherwise it's pot luck rather than a challenge.
It doesn't bother me so much because I have the completionist achievement, but my mate who I play with, he doesn't, so I try help him get it, but when this comes up and the stupid thing doesn't even poke his head out, is annoying.