The huge water elemental grapples the PC, and pulls him into the murky water. The rest of the party manages to finish it off with ranged attacks and spells, but oh no! It was a load-bearing elemental, and when it dies the cavern starts to collapse! A great big rock falls from the ceiling and hits the now-submerged player, pinning him to the pool floor. The party dives into the Devilfish-infested waters in a bold attempt at a daring rescue.
Let me start by saying I'm all for hand-waving for the sake of some good action and an intense encounter, especially if it throws the PCs into an environment to which they are not accustomed.
With that out of the way, is this situation possible to construct in Pathfinder in a completely legitimate way? Certain things certainly don't have to be explained by the rules, such as a cave-in. It's entirely possible that just happens naturally, without having to be explained by magic or a trap (but bonus points if it can be worked in somehow!)
- Assuming the cavern can be made to collapse, what happens to the PC? Does the falling rock make an attack against the PC? A CMB check? Does the PC reflex save? Swim check?
- Assuming the rock can be made to "hit" the player, does he just take damage, or is there a way to have it apply the grappled, or even pinned condition to the player at the bottom of the water? Is the player helpless?
- If so, what means can the player use to escape? Strength check? Escape artist? CMB? What is a rock's CMD?
- Can the other party members assist in a meaningful away? Assuming they don't have any magic that "solves" the encounter like turn rock to mud etc, what can they do if they've dived down to the bottom to try to help him escape? A measly +2 from "assisting" on some check doesn't seem that great, when realistically 3 people all shoving a rock should be 3 times as effective, if not moreso, than one person (who is trapped beneath it).
I appreciate that the answer to this question is "probably not," but I'd still like to see some attempts at legitimizing this action sequence.