This is the Special Commissions screen that has the special missions where you can spend AP to obtain reports or Credits. The top button will take you to the missions where you can get EXP reports and the bottom one will take you to the missions where you can obtain Credits.
The Ruins stage is an indoor stage where you will face swarms of little robots with heavy armor. Frontline units other than your tank(s) are not recommended because they can and most likely will run ahead of your tank, get swarmed, and die so try to limit yourself to 1 or 2 tanks at most, and the rest AOE damage dealers. Tsubaki, Hoshino, Sumire, and Marina are good tanks for this stage while Iori, Junko, Nonomi, Momoi, and Cherino will be great damage dealers for this mode providing great AOE damage. Hifumi also has decent AOE damage while also taunting and grouping the bots using Peroro-sama which can give your team a chance to breathe and heal themselves while the swarm is distracted by Peroro. Serina and Kotama are chosen because of their low cost heal and low cost attack buff which allows for easy cycling to the skills you actually want to use.
During the Slumpia Square stages, there will be a boss that must be defeated within 30 seconds. When the boss is defeated, it will respawn with more health and the timer will reset. The boss can be defeated a total of 5 times and for each time, it will drop credits ranging in three values. The odds of these values are as follows (Average is assuming sweep/fully clearing stage):
Credits to Noxy#8299 and アイザヱー#6944 for the drop rates data