My players had a rough time against the quasit, so I nerfed her and fudged the rolls to keep the fight from dragging on for ever. With six players, 5 rounds of combat against the same creature will make everybody bored.
When they stunned her and she dropped her dagger, they picked it up and dropped it in the runewell. I rewarded their ... willingness to "try something, anything" by removing her damage resistance.
Still, they basically relied on the lucky roll to hit her, and I finally just fudged the fighter's grapple to make it work, while "forgetting" that she can turn invisible.
This encounter was frustrating on both sides of the screen.