Once I had a nice ambush of cannibalistic halflings (war2/rog1 plus a ftr2/rog3 leader with TWF kukris, a bard2 with Tashas and a pearl of power 1 and a wiz3 with color spray and some other knock out spells). The ten players were level 5-8, the halflings were 6 plus the three leaders IIRC. They started disguised as playing kids... and started from flanking positions. Due to their tumbling and horribly good to hit rolls (dex 16, small size, Weapon Finesse, bards song with that level 1 spell for +2, flanking +2) that ambush nearly ended as TPK even though the halflings didn't concentrate on single players to knock them out fast. Not one player failed his save against the save or die spells... still the flanking rogues put them horribly fast into the single digit hitpoints and the cleaving fighter never came close to attacking more than 1 halfling... since they always moved around to flank the next victim. Perhaps scouts would be even more devastating.
A Bless spell would have helped too... perhaps a wand/scroll for the leader with UMD? And you can always use these halflings with poison.
Edit: By the way... kidnapping a party is usually not a good idea... most players hate it. Give them a fair fight and let them think they just screwed up and you're generous not to kill them but you're looking for a emergency exit to avoid a TPK. And if they win by pure luck... let them win.
