Just like in the original Final Fantasy Adventures, there is a quick way to get the ultimate armor in the Mana Fortress. First you must fill your regular item inventory then kill the corresponding monster below for the armor you need. When a chest drops, you will most likely get the "Can't carry anymore!" message. Just keep examining the box and it will eventually give you the armor.

Demon Head- Samurai Helm.
Elephant- Aegis Shield.
Ninja monster- Samurai Armor.