Besides wg stage 1, the other milestones have different requirements. For example, for Genma stage 2, have her be transformed while attempting to change floors on floor 1 and 3. Genma currently only has 2 stages (1 wg, 1 be)
Currently, the only unit that doesn't have stages in ch1 is Yriosa.
Other characters are found by first finding an rare item in the Beginner Dungeon, but its drop rate increases when at least 1 of each mimic has been defeated. Once obtained, after resting a day, you can begin using Soul Stones in a new way (feeding) through the Soul Stone menu
After recruiting the unit that requires feeding 20 Soul Stones, you enable Razzle wg progress through the Soul Stones she normally asks for (recommended to do this before making any floor progress in the Exceed Dungeon)