I would think the mexican stand-off would work best when the PCs get their first, and the other parties arrive just after, or if all 3 approach at the same time.
On an open field (your crater), this may be a harder thing to do. If the area had LOS blockages, the bad guys could say something witty as they come around the corner, thereby keeping the game in RP mode, rather than combat mode.
It's further complicated becasue you have a PC party, making it just as likely the PCs will attack both bad guys simultaneously (after all, they've got multiple attacks in the round).
You might make the macguffin react to violence, giving the PCs a very finite time to get the job done and extricate it, once they start fighting. Make sure they know this ahead of time (and all the bad guys know this too). This will make them all edgy, and wary to start the combat. But when it starts, expect no-holds barred.
Janx
On an open field (your crater), this may be a harder thing to do. If the area had LOS blockages, the bad guys could say something witty as they come around the corner, thereby keeping the game in RP mode, rather than combat mode.
It's further complicated becasue you have a PC party, making it just as likely the PCs will attack both bad guys simultaneously (after all, they've got multiple attacks in the round).
You might make the macguffin react to violence, giving the PCs a very finite time to get the job done and extricate it, once they start fighting. Make sure they know this ahead of time (and all the bad guys know this too). This will make them all edgy, and wary to start the combat. But when it starts, expect no-holds barred.
Janx