In North America the mountain lion or puma is the top predator and has no enemies other than the bear (perhaps), humans (certainly) and another mountain lion. During dispersal when a young adult puma goes in search of his own home range (a “residency”) he enters a phase in his life which is the most hazardous. Resident male pumas are responsible for “most of the mortality among dispersers.”
For example in Florida two dispersing males and one transient male (a male without a home in effect) were killed by a resident male. There are a number of studies on resident males killing subadult and yearling pumas. This is called “intraspecific strife”.
Humans kill pumas for entertainment. I suppose we have to regard humans as the puma’s enemy both because of sport hunting and because humans grab the mountain lion’s territory for housing and industry.
Traffic is also an enemy of the mountain lion. Florida is particularly hazardous for pumas crossing freeways.
In Mexico, Central America and South America the jaguar must from time to time be regarded as an enemy of the puma as their ranges overlap but information is scarce. These two big cats avoid each other in the interests of survival. The jaguar has superior strength and size. One record exists of a jaguar attacking a puma and killing it (Howard Quigley).
Young pumas are pretty vulnerable. The picture shows young pumas being stalked by coyotes.
References: Myself, Wild Cats of the World.