This one was a bit annoying to get the right combination. Feel free to improvise. This is to ensure you get the main reward. I wasn’t able to get 3* (yet).
Guardian Stone: (6) Frost
Break Griesel 1/1
Use a defense type hero 2/2
Use Iris 1/1
Clear in 60 turns
Formation: Emphasized Front Row
(All Non-FC except Iris) Degas (Ice/Defensive), Rera (Nature), Leger (Healer/Revive), Uloom (Defensive) and Iris.
1st Wave should be used to prepare the party for the boss fight. Try to reserve special attacks for the second wave.
Degas should use [Aegis of Faith/2nd skill] Iris should use [Origin of Protection/2nd skill] to shield all allies and Leger should use [Realm of Recovery/2nd skill] or her [normal] attack. Rera should switch between [Verdant Fire] and [Summon Uloom] to wipe out all enemies.
2nd Wave: Boss Fight
Griesel has the chance to KO a unit in one hit but she seems to target attackers, mostly— so just be ready to revive Rera. If she KO’s the healer (rare), you will need to restart the battle.
Stall the minions as necessary.
The boss heals but not a significant amount.
Rera and Uloom are to be used as DPS, executing special attacks targeted at the boss. Degas switches to [normal/special] attacks to start breaking all 6 guardian stones. Iris needs to use [Origin of Protection] to shield all allies while Leger needs to heal/revive as needed.
Once Degas’ breaks all 6 stones (she should be alone by now), switch Iris to her offensive attack [Inherited Power]. You can switch Leger to offensive as well. By breaking Griesel, you should be able to meet all the conditions of the mission. You might need to break her a second time to speed up the battle.