grass
Yes. But hunting down an adult hippo is not practically possible for a lion. Though, it may hunt down young hippos that stray off from their family
yes
Hippos, though naturally prey animals, are too large to be actually hunted and killed by top predators like lions, leopards and hyenas. However, a dead hippo will be eaten by these predators as well...
sarpent (a large snake like reptail like pithon) eats a hippopotamus.