Perhaps a rival adventuring company cuts a deal with [insert nearly any sentient monsters] to raid the campsite while the rival company enters the dungeon. You can have supply wagons, animals (even better if you're in a desert, camped at a small oasis, and the animals and supplies are the difference between life and death) and a central campfire. The adventuring company might also have a "tag-along" NPC, someone important who is a [important prisoner they caught along the way / a noble benefactor funding the venture / a princess pretending to be the camp cook in order to escape a pre-arranged marriage]. One of the wagons could have forms of healing and a Raise Dead scroll (the party isn't high enough level to reasonably use it, but it's a potent treasure].
You can then create perhaps 5 scenarios, with at least half (2) involving non-combat, and a pool of bad guys to carry them out (subtracting losses as you go). I'd have at least 3 distinct bad guy types (e.g. ranged, melee, a single caster). They need to ideally be cowardly or cautious so that their greater numbers still don't encourage them to bull rush the camp (e.g. if they lose 25% in a single attack, it's a disaster and they flee).
Issue #1: Enemy distracts with skirmish (disengage) attacks on one side of the camp while another moves to set fire to a wagon that doesn't look valuable (e.g. sleeping quarters). [You'll need a set of skill checks or the like to put out a fire and a solid consequence for failure that impacts the rest of the adventure]
Issue #2: the campfire spooks pack animals. Some of them take off. Those with skills in tracking or animal handling will be needed. Enemy sends a few troops to try and take the loose animals.
#3: The NPC is recognized for whatever reason as important and they try to have some Sneak into the camp to snatch and grab while others harass.
#4: The enemy manages to divert a monster (by lure, by harassing it, making a deal, etc) into the camp (a CR 1 maybe). It is irritable but not necessarily looking for a fight. Its presence will cause havoc as enemies try to invade a wagon and make off with goods.
#5: Grand finale. Whatever is left makes a move on the camp but flees after 25% casualties. If you're at an oasis, use all sorts of subterfuge like some of them trying to stealthily swim or hide behind a tree, etc. You need some to flee into the dungeon (after their benefactors).
Afterwards: the higher level group uses a spell to send a message to the party. They were attacked by a rival group and one of their members has been slain. One of the enemy has escaped with something valuable and is probably headed their way. The party is given permission to use whatever healing supplies they need if they haven't already. And they give the command word to a home-brew item, a stone called the "refresher" that restores energy as if taking a Full Rest. They recommend entering the dungeon lest the foe escape.
Now you can stock your dungeon with survivors of the raid as well as any monsters or traps that the adventuring party might have bypassed. Have an open enough plan that it's going to be hard to cover all exits the enemy could take.
Once the party has succeeded, you could have a scenario where the group needs the Raise Dead scroll (if it survived the raid), and the players might have to run/sneak past some hazards or solve a puzzle with VERY limited clues due to the limited nature of the communication spell being used.
