Set a thief to catch a thief.
If you want to know ways animals can kill naked people in the woods, there are many. A lone naked human is barely a match for a jackal. But as for our species generally, animals were no match for even our distant ancestors. We have many ways to compensate for biological weaknesses. That is why the most efficient killer of humans is humans. The intelligence, creativity and ferocity of humans is largely a result of millennia of intrahuman competition - and cooperation.
The nature of the engineered human depends on the tech level available. If you have guns, then small, resource-efficient humans would be best because you can have more of them and have them not starve. If it is earlier tech then a variety of sizes makes sense - large ones for your shock troops but not so many they eat all the food.
For purposes of fighting, most human weaknesses have been circumvented to some degree with tech. You can shield soft parts, cover soft feet, make fire to compensate for lack of fur, domesticate dogs to watch for you while you sleep. You can preserve and store food to compensate for how you are always hungry.
Ideas about how humans could be more efficient killers of non-self group humans.
Target group humans are not acceptable as mates or slaves. Neanderthals were not wiped out, because they are us. There was only one way that could happen.
Killer group humans cooperate flawlessly. The problem with fierce humans is that they turn on each other. Not your bred killers.
Killer group killers don't kill their own kids! Don't kill the kids! Infant mortality at the hands of unrelated males (in present society read boyfriend, stepfather) is a big problem even today.
A weakness of humans is that we need resources: food, water, fuel. Modern warfare goes after these resources without compunction but that is a more recent development. Pretechnological warfare spared resources that the victors would need for themselves - e.g. Deuteronomy 20:20
Only the trees which thou knowest that they be not trees for meat,
thou shalt destroy and cut them down; and thou shalt build bulwarks
against the city that maketh war with thee, until it be subdued.
The human killers have no such compunction. They make scorched earth warfare. How they get away with it is a matter for the story. Maybe they can eat dirt.
Focus. The humans killers are always coming back to the thought of killing their target humans. Real humans over history get tired, get hungry, get horny, get cold. They want to take some scalps and then rest up. Industrial type total war is a very recent development. Your human killers might be primitives but they have the total war focused mindset - it is all about killing, all the time.
The biggest weakness of humans - we are noncombatant tiny kids for many years and weaklings for many years after that. Your human killer humans grow up very fast. At age 3 they might not be full grown but they are coordinated and capable.
I will have to read After Earth. I am not sure why technological savvy humans fighting hungry large carnivores don't just poison and trap them all, like we do. It does not sound like these things are ghosts, or live via photosynthesis.