Magic isn't necessarily a "weakness" for Superman, more of a vulnerability. He can be tricked or trapped by magic just like everyone else.
Anyway, Superman's other major weakness besides Kryptonite is red sunlight, which depowers him. Under a red sun or sun lamp, Superman becomes just a regular man. He would still be strong, but only at peak human strength, not superhuman strength, and he would have none of his powers (flight, laser eyes, ice breath, impenetrable body, etc).
Regardless, there are people in the DC universe that are similar in strength to Superman and can beat him in a fight (or come close), such as Darkseid, Mongul, Lobo, Parasite, Doomsday, Daxamites, evil Kryptonians (Zod and his followers), etc.