When Elephants are microed, Elephants win easily.
When neither are microed, Elephants win easily.
When Prodromos are microed, it's a tie on average.
So in-game, unless you can for some reason devote much more time to battle micro than your opponent, fully upgraded Prodromos will not counter Elephants. By the way, Poseidon provides no unit stat boost to cavalry, he only makes them cheaper.
Take my love, take my land, take me where I cannot stand.
I don't care, I'm still free, you can't take the sky from me.
Take me out to the black, tell them I ain't comin' back.
Burn the land and boil the sea, you can't take the sky from me.
There's no place I can be, since I found Serenity.
But you can't take the sky from me...