Slobadog
King
I myself think that that Ballista Elephant is amazingly easily useful. Horses are fairly common and the AI always sends in mixed stacks. Lure the enemy into you territory and wipe out its invading stacks. Since they were only battling mounted units the Ballistas will always survive and will gain easy XPs. These easy XPs will lead to easy promotions that will make an already powerful unit even better. Then go on the offensive with your newly super Elephants. Almost never fails. Unless your opponent has elephants itself but settlers or raiders should deny them that. Just don't deny them horses.