I have the party facing a hydra. it looks like they are not to hard to kill if everyone goes for the body. but i thought if it was in a tunnel only as wide as it was they couldn't hit the body and would actually have to hit the heads. the question is what if they could enmasse bull rush it back into it's cave so they could get to the body. it says bull rush is only 1 size larger, and this is huge. so, if they get desperate to push it back as a tactic, could they do it enmasse. i like the idea of them working as a team. they are not rules sticklers, but i'd rather not take to much liberties with the RAW.
I believe grapple is the only maneuver with RAW for multiple creatures working together. You'll have to houserule this if you want them to attempt it.
But could you explain why you think they won't be able to hit the body while in the tunnel? Body parts don't have particular locations in PF, in the sense that it is not the case that the heads are between the PCs and the body. The whole hydra is just sort of there in all its squares, any part of it hittable by anyone.
Or you could just cast enlarge person on the strongest character (bringing them to Large, within one size category of Huge), then have the enlarged character bull rush the hydra while the rest of the characters use "aid another" (adding +2 for each one that makes a successful CMD check vs DC 10).