Start off by clearing the room of Heartless and striking the door. Synthesize using a card with a value of 1 or more. Go through three more rooms of clearing Heartless and striking doors until you reach Room 5. Use a Gimmick Card and the Key of Beginnings when you strike the door.
Guard Armor is an easy boss that won't need any special Dark attacks to beat. Attack the arms and legs until it reaches its second form, which should still be a breeze. Guard Armor is split into five parts, a body, two legs and two arms. To defeat this boss, all parts must be destroyed. Focusing on either the arms or the legs, take out part of Guard Armor using simple Attack cards, healing when necessary. After all the arms and legs are gone, go for the body. Watch out, for the body spins around, seemingly randomly, making this part a little tricky. Break the enemy cards to make this battle simpler and to allow you a couple hits. Continue to heal as necessary until the boss is defeated.