Meat & Drink (CR 5)
Story
This encounter doesn't have any particular story, just two monster groups working together in symbiosis, specifically ghouls and stirges. The stirges get the blood, the ghouls get the flesh. This could be an incidental encounter in a swamp or graveyard, or the basis for an adventure hook.
Tactics
The stirges attack first, softening up the party and providing a distraction (+5 to the ghoul's stealth check) for the ghouls to sneak up on them. This is particularly nasty because the constitution the stirges drain will make them more susceptible to the paralysis effects of the ghouls. For additional options, the location could be a swamp (incurring difficult terrain and waters up to chest deep & even Lacedon ghouls) or an abandoned graveyard (which provides plenty of cover and difficult terrain via headstones). To add to the effect, a light mist (concealment for those not adjacent) could further augment the combat for both sides.