Just ran this encounter for my group this past weekend. Everyone in the group has a minimum of 20 years gaming experience, the last 15 of it together. Same group TPK'd out of Kingmaker and completed Carrion Crown. Good group, but still one that makes their PCs more for flavor and consistent concept than from a min-maxing perspective.
Human Fighter 2 (sword and board)
Human Magus 2 (whip magus)
Human Rogue 2
Dwarven Cleric of Torag 2
The PCs were pretty seriously tapped out by the time they got to the Cathedral, having chosen to explore the entirety of the Catacombs first. Since they opted to return to Sandpoint and rest for the night before continuing, I gave the quasit an additional Sinspawn who began the fight hiding under one of the staircases.
Encounter opened with her summoning the second sinspawn from the well during the 'surprise round.'
Round 1. Magus casts Enlarge Person on the Fighter. Rogue nails the quasit with a cold iron arrow from Tsuto's old shortbow. Enlarged fighter and cleric move to engage the two sinspawn. Quasit goes invisible.
Round 2. Party tussles with sinspawn and falls prey to some annoyingly bad dice while quaist starts casting Summon II.
Round 3. Summon II triggers and spawns 3 riding dogs to flank the PCs with the sinspawn. Quasit starts casting Summon 1, still invisible. Party starts panicking a bit as they are now outnumbered, injured, and throwing terrible attack rolls.
Round 4. Summon I triggers and spawns another riding dog. Quasit becomes visible as she drops a Cause Fear burst on the party. Everyone saves but the rogue, who takes off. Not knowing how long the summoned dogs will remain, the cleric calls a retreat and the group begins a fighting retreat out the double doors.
Round 5. Fighter goes down while heading for the doors. Quasit fails a Hold Person on the cleric. Cleric of Torag, now fully buffed up, puts his back to a wall and holds off the remaining dogs while channeling heals for the retreating party.
Round 6. Remaining first 2 dogs unsummon, causing the party to reassess their options and re-enter the combat. Dice finally turn and they dispatch both sinspawn. Rogue returns, the CF having timed out. Quasit, now missing most of her spells and with all of her followers dead, goes invis again to reassess her own options.
Round 7. Party closes the doors to try and cut off the quasit's escape, but have difficulty coming up with an idea to draw her out.
As the GM I am loathe to have her begin suicidally throwing that ineffectual dagger around now that her spell load is mostly gone and her followers/summons are dead. I know she's a bit paranoid and twitchy, but she's also thousands of years old and smart. She sees that the party has an enlarged fighter (with a ranseur for extra reach if he needs it), a deadeye shortbow with cold iron arrows, and a magus with a whip. I don't want to cheat the party out of a hard-earned victory, but neither do I want to hand them one too easily because the quasit is resource-tapped and 'decides' to foolishly engage rather than retreat to lick her wounds. So she sits silent and invisible, intending to at least wait out the Enlarge while she considers her options.
So the dwarven cleric decides to make her angry and draw her out by advancing and urinating in the runewell.
We went over time at that point, so will be finishing the encounter next session. Though I am a bit unsure as to how I should have the runewell react to the dwarf's defilement. I feel that hitting him with the same effect as if he'd entered the runewell is probably appropriate. Moreso than having it summon a sinspawn, which I interpret as being a more 'directed usage' of its power. The group only half-grasped the clue about the runewell fading, so I am hoping that if I drain another 3 points off of it by having the dwarf enrage, it may prompt them to burn its remaining points and thereby deactivate it.
Of course, the quasit showing back up, even spell-deficient, once the cleric of Torag starts raging around (assuming he fails his save) could make for an interesting conclusion to the encounter.