I agree that this is not a tested of bounded accuracy. By placing additional restrictions on the encounter building, you're negating the stated premise.
However, if you want to talk about challenging a 10th level party with an encounter of (per book - no modification) CR 1 monsters with a maximum of 8 monsters, you're likely going to want deception in the situation, or encounter control.
A Pixie and a Dryad Watch the PCs from a distance, with the benefit of superior invisibility for the pixie and pass without trace for both, their stealth bonuses are +17 and +15. They wait until the rogue or fighter is on watch and then the dryad will charm them - silently - and invite them to go off and frolic. They offer up their Changeling friend as a suitable replacement for the guard duty, with the changeling in a suitable form to be a capable guard. Once out of earshot, the trusted dryad will instruct the PC to go with the pixie to see the dryad's treasures and joy, with the pixie casting fly on the rogue or fighter and then leading them up into the sky towards a nearby very highly elevated spot. Once the PC is 200 or more feet above the ground, and above a hazardous terrain if possible, the fly is dropped and the PC falls taking 20d6, or 70 damage. A rogue with a 14 con would have 73 hps. A fighter 84. Either would then by a valid target for a sleep spell which can be delivered during a surprise round. Once asleep, and without allies, either would be easy to finish off.
You now have a cleric, wizard and either a fighter or rogue still left in camp, asleep. The Changeling waits for the dryad to return with the equipment of the deceased PC, freshly cleaned, and dons the equipment of the deceased PC. The dryad then silently wakes the rogue/fighter that remains and a second pixie repeats the method of offing the first PC.
If the other PCs awake during the night, the changeling is there to put them at ease in the guise of the PC that was on watch.
You're down to a sleeping wizard and a sleeping cleric. Both are unarmored. Their ACs should be no greater than 12 (maybe 15 is a wizard has mage armor), and they're being attack with advantage in a surprise round. Assuming there is no mage armor, the wizard would be the target of the 4 shadows that attack in a surprise round, as they'd have the greatest chance of killing the wizard with the strength drain. With advantage, against AC 12 - You're very likely to kill that wizard in the surprise round, but if not you have a good chance for a few of those shadows to go again during the next round before the wizard and finish it off before it can rise. You could also have some
Shadows can attack silently, so there is a good chance the cleric did not wake up and that the rogue died in their sleep or before they could raise an alarm for the cleric. That allows the Changeling, 4 shadows and dryad to all do a coup de grace style attack and try to finish off that cleric before it goes. Right after those initial attacks, the 2 pixies hit the cleric with phantasmal force spells convincing the cleric that it is buried under a giant boulder and is being crushed by it, slowly. Intelligence saves of a cleric are likely very low. The wizard is going to lie there, trapped by the 'boulder' unable to see the shadows to identify them for purposes of determining a turn undead would be helpful.
You get a decent chance at a TPK.