1. Speak with Calise and read the Instructions.
2. Carve these words upon your soul: Heavy Units beat Skirmishers, Skirmishers beat Support Units, Support Units beat Heavy Units
3. Activate The Ring of Fate Ritual, this will give some shielding for mistakes. I don't know if having the Healers activate counts as losing someone for the achievement, best to play it safe
4. During the actual mini-game, you will be subject to a series of events that occur in mostly no set order. You won't be directly told what each unit is, but you can guess. A noticeable phrase or part and what Unit to use is listed below, but in general mages are Support Units, Archers and Ambushers are Skirmishers, and Everything Else is a Heavy if you wish to try to do it yourself.