I played around with two serpentines against a elephant unit in a custom battle. Hell yes they work. Dropped 3 eles with first salvo (that's 3 out of 4 rounds hitting from 1/2 the map away!). Later volleys were quite as good.

Serpentines are kinda hard to get though. The incredibly low tech arty solution to eles for any faction is the ballista. Turn to flaming shot and see some elephants fry. Other more advanced arty is more inaccurate and one ballista bolt is instafrag anyhow.

Note that rocket launchers work very well but ribaults not quite as well against eles. Haven't tried monster ribaults yet. At any rate, don't use horse archers against them. Simply a giant waste of ammo even if you take em down. Furthermore, the morale impact on horses by elephants is huge. Even my triple gold siphais routed from Timurid eles.