The Cradle Guardian is encountered B3F of the Echoing Library, guarding the Imperial Prince.
The Cradle Guardian starts out with no weaknesses or resistances in its Phys plating. It will open with two randomly chosen attacks, unleash Arm Mixer on its 3rd turn, then change its plating on the turn afterwards. Its plating pattern is always Phys → Fire → Ice → Volt → Phys. Fire plate is weak to ice and resists all other attacks; Ice plate is weak to volt; Volt plate is weak to fire. After it has changed its modes, it will choose a random attack, then use its elemental Mixer, before changing modes again. Once it finishes the loop of modes and reverts to the Phys plate for the first time, the timing of when it uses Arm Mixer and when it changes modes become more random. The second time it reverts to Phys plate, it can even begin to change modes without using the Mixer attack.
A party member with Runemaster skills will be vital in adjusting the party's resistances, or a Fortress' Strike Guard and Element Guards will serve the same purpose. If Logre is in the fight, pay attention to the Cradle Guardian's elemental affinities and when it changes modes to make the most of his elemental Drive skills.
Once the body is defeated, the Guardian Head remains, and begin counting down 3 turns to a self-destruct. It is weak to all elemental attacks and does not retaliate, so hold nothing back to deplete its HP. If the countdown is about to expire and you are nowhere near defeating it, just have the party defend on the final turn (and hopefully well healed) to prevent a wipe.
- Arm Mixer (uses arms): Ranged cut + bash + stab attack to entire party. Phys plate only.
- Flame Mixer (uses arms): Ranged fire attack to entire party. Fire plate only.
- Ice Mixer (uses arms): Ranged ice attack to entire party. Ice plate only.
- Volt Mixer (uses arms): Ranged volt attack to entire party. Volt plate only.
- Megaton Punch (uses arms): Melee bash attack with splash effect.
- Pile Bunker (uses arms): Melee line-piercing cut attack.
- Dis-Element (uses head): Increases elemental resistance for 5 turns.
- Electric Shot (uses head): May paralyze the entire party.
- Restraint (uses head): May inflict head bind on the entire party.
To get its conditional drop, the Dream Carapace, you must defeat the Cradle Guardian while it is asleep. This is one of the ingredients for making the Hihiirokane, the best armor for Bushi. If you use a Formaldehyde, it must be in the same turn you destroy the Cradle Guardian body, as the Guardian Head counts as a separate entity.