Amoonguss would be an issue if Keldeo doesn't have any HP/Icy Wind to hit him as that shroom also wall some Terrakion versions, but I think your Keldeo was supposed to carry it anyway.
For another Terrakion partner, Celebi works fine, he can tank what Terrakion hates with his decent bulk like EQ and Fighting moves, hit hard with Psychic/Leaf Storm the classical Terrakion counters, U-Turn to lure Dark atack to Terrakion and deal some extra damage against the Psy pokemons.
And there are many other options such as T-Wave, screens, SR, etc.
Hope it helped a bit!