Jackals and Hyenas are crazy, scrappy, tough things that will go after almost anything.
I think I read once that a Water Buffalo has no natural enemy in its immediate ecosystem, but that seems wrong as I have seen videos on Youtube of different animals fighting them.
A fully grown, African bull elephant in a really pissed off mood is difficult to defeat. They like to squat down and push their tusks/head onto their victim, crushing it between their head and the ground. I don't know what *single animal* could give such an animal a fight.
If packs are allowed, a whole bunch of female Lions can probably kill anything. There are videos of them taking down elephants.
Then you have this crazy Tiger in one of my favorite animal videos ever! I think he'd put up a tough fight against anything, but there are plenty of mammals that would take him down.