Forcing a Dismount

[Tarma]

Over the weekend I was playing in a PFS scenario which involved combat with someone on a mount. They were really the major threat in the battle, and needed to be taken out. However, we weren't doing enough damage to kill him in one round and killing the mount would have taken even longer.

Is there any combat action that can be taken to force someone to dismount from their mount? I know there's a feat that allows it, but that's only if you're also mounted.

[Eragar]

I don't know of anything in RAW, but if I was GM'ing it I would allow bull rush, grapple, trip (On the mount), or reposition. Albeit at a penalty.

[Zotpox]

Polearms spesificaly the halbard is the right tool for the job however a set spear will do the trick.