Monsters of the dungeon could develop immunity to attacks at times. As such, there were three categories of attacks that were devised to address this problem. The usual melee attacks, long-ranged attacks, or spell attacks.
Boss monsters above the 10th floor, where the "real" dungeon begins, usually have at least one type of attack immunity, but most often, it is missile immunity. As such, boss fights usually end up with melee up front and long-ranged mages behind, while weapons like crossbows, bows, or guns are rendered completely useless.
This time, however, the mutant rogue boss, the Red Cave Spider Mother, was immune to spells, a rarity even in the first 50 floors. As such, they were supposed to be facing an uphill battle, but...
"Alright! The air seems fine, but wear makeshift facemasks just in case! When you feel dizzy, alert your allies and retreat here immediately!"
