The lack of opposable thumbs would significantly limit their ability to craft tools. Even if they could use trunks to craft melee or throwing weapons, there is no way for even the most intelligent, coordinated elephants to overcome a ditch with a line of ballistae or cannon on the other side. Cavalry were able to deal with this due to their relatively thin profile and high maneuverability, and even then ditch + spear wall + artillery or ditch + grape shot is a pretty effective counter.

It would be interesting to see them execute the Fabian strategy though.