I was reading recently the top ten reasons encounters get trashed and among them was placing the villain in front of the 200 foot drop. It invites the bull rush and the end of the encounter.
Well, putting him right at the edge is just asking for it. It's much cooler to put him away from it, and have the PCs knock the BBEG over the edge on their own.
I'm not sure how it would work if they villain responded with a grapple? Is it even possible?
The only way it would be possible is in regards to an attack of opportunity, as you point out below, or if the villain had readied an action.
It makes sense in real world terms (I know this is fantasy). However, there is a vicious cycle between the bull rush causing an AoO then the grapple causes an AoO and so on.
The thing is, there's no "and so on" at the end of that sentence. Presuming the bull rusher doesn't have Improved Bull Rush, the villain doesn't have Improved Grapple, and that neither has Combat Reflexes, the way things will go is like so:
1) PC attempts to bull rush the villain.
2) BBEG gets an attack of opportunity in response to the bull rush attempt; he uses it to make a grapple check.
3) PC gets an attack of opportunity in response to the grapple check.
Now, for the AoO the PC gets in step three, he could in theory take some action that itself provokes an attack of opportunity, such as making a sunder or trip attempt, but this ignores that the villain, having only one attack of opportunity per round (remember, nobody here as Combat Reflexes), can't take advantage of it. Hence, there is no "vicious cycle." Besides, it'd be silly for the PC to do something else at this point, since he's already trying to push the guy off a cliff.
I think that the AoO must be done with an "armed" attack (i.e. something that doesn't cause an AoO, any of the improved feats solve that problem of course).
I don't believe there's anything in the RAW about that.
Hopefully my question made sense. I know the DM has final say on the rules, but the DM doesn't want to fall into situations where sometimes a tactic works and other times it doesn't.
But that's the reality of things - sometimes a tactic works, and sometimes it doesn't, whether due to bad luck, preparedness on the part of the enemy, or something else altogether. No tactic works every time.
In the instance that the BBEG makes a successful grapple attempt (making the initial touch attack, followed by winning the opposed grapple check), then things become a bit iffy. The rules for a bull rush don't expressly say that a successful AoO ends a bull rush - as such, you can still push the target back 5 feet or more. However, you're still grappling (and, I believe, the one who initiated the grapple doesn't move into the other character's space until the subsequent round). Hence, you most likely end up grappling, even though the PC pushes the BBEG back.
This could very well result in a situation where the characters end with the PC on solid ground at the edge of a cliff, the villain one square
over the edge of the cliff, and both still grappling. That's a situation not covered in the RAW, but I'd say that both characters go tumbling (possibly with a last-ditch opposed grapple check from the character on solid ground to get out of the grapple in time)...which'd be damn cool.
