This scenario is suitable for any monstrous encounter within ruins, either above ground or below, and fits a wide variety of map types. The setup below describes one such battle map, but feel free to substitute your own.
The scenario begins with the PCs on a raised platform in the center of a ruined chamber (borders are Height 1 from the surrounding sections); the monstrous creatures emerge as the players reach this platform. The chamber is 3 sections across and down with the exits on the north and south sides of the room. Stairs climb down the platform to the south, but the stairs on the north side are broken; climbing up or down them requires an Acrobatics vs. 1d4; failing results in falling Height 1 to the north section, suffering 1d6 falling damage.
This scenario is appropriate for any lighting conditions. It could be open air daylight (or nighttime) with the chamber's ceiling long since crumbled away. Or perhaps a small hole in the ceiling allows a bit of light through, illuminating the central raised platform and leaving the rest of the chamber shadowy.
Corner movement is not allowed. To simplify, draw the center section slightly larger than the surrounding sections to eliminate corners.
0 0 0 0 0 1 0 0 0 0 0
These Adversaries are not particularly intelligent and will simply go after the nearest enemy. The Insect Clouds will avoid any dark sections (if possible), while the Serpent Vine prefers shadowy light, and the Grime Maw will try to stay in any dark or shadowy sections. All Adversaries will avoid the Insect Clouds (including the other Insect Cloud).
Victory allows the PCs to continue exploring the area. A reward of a few Valuables may be appropriate, such as one per PC.
If the PCs are defeated, they're forced to turn and run for their lives. They leave the area, each suffering 1 Injury in the process. PCs may regroup and try again.
For added difficulty, make the chamber partially flooded; the N, W, and E central sections require swimming. The Grime Maws and Serpent Vines don't care for water and will try to avoid fighting in it, but the Insect Clouds hover over it and thus take no penalties. If you want to make more of the chamber flooded, consider swapping the Serpent Vines for Bog Crawlers (1 for 2 since the latter are mooks), as they have the Aquatic keyword. Or, for a potentially bigger threat, swap an Insect Cloud and a Grime Maw for a Silk Spider.
If your ruins are in a colder climate, you could swap out two Grime Maws for an Ice Mote (or, with three PCs, the Grime Maw and Insect Clouds). You might also swap an Insect Cloud and either a Grime Maw or a Serpent Vine for two Snow Hounds. Any remaining Insect Clouds could be swapped for Ectoplasm Bulbs. If you want to remove all Serpent Vines and have one left, you could switch it for a Bone Shrine, positioned to give an advantage to the Ice Mote or Ectoplasm Bulbs. In this case you might adjust the PC starting position so it's harder for them to get to the Bone Shrine's section.