The enemies on this map are mostly Undead or Spirit armor type. Fire/Holy/Omni weapons and attacks are mostly useful. Note that omni type isn't effective against undead but deals decent damage vs. spirit types. Fire is good against undead and decent against spirits. Holy attacks will eat everything on this map for lunch and is the best choice of element for clearing this map.
The B rank magic skill is also extremely useful in this stage, teleport anywhere you control. Colette with Move again abuse also works if you don't have the magic yet or lacking the mana to cast it. She can even equip the Fire dagger to easily dispatch any enemies she encounters here.
After you have closed all the Monster Vortexes, a new area opens to the South. Have someone ready to defend.
After reaching further the room with two treasures, a group of 3 ghosts and 1 treasure box appear above the undead worm spawn in the North area (middle spawn). They also spawn on turn 15 if you have not passed this room by then.
Chest behind the bronze door contains a Creation Fluid (創造の錬液を).
The southmost of the second set of monster portals has a chance to spawn Treasure Putetetto! You may want to hunt a few here for their valuable drops. However, enemy spawn rate is suppressed (or completely prevented - unconfirmed) if you control enough territory, so getting them to spawn might be kind of difficult if you're playing it safe and keeping ~90% territory under control.
A chest with fluid of decay appears when you replay this map.