Increase a character's armour class (AC) to -15 or less.
The easiest way to do this is to create a Thief character with the Blade class kit, and set them up with 18 Dextrity, which will give them -4 AC. During the main game, acquire and equip them with Spirit's Shield+2, cast the Lv4 spell Spirit Armour, and perform the Blade's Defensive Spin action. All of this should reach the required armour class.