Infernax

Here's a written guide:

1. Beat the fourth boss, and unlock the air dash ability.
2. Go to the site of the third boss battle, but don't enter the castle.
3. Dash to the right across the ocean, and go in the secret dungeon.
4. Play through until the mini boss battle and win.

- He will stretch his tongue out and jump around, typically jumping twice, and landing. When he lands, a purple/red flashing vine appears around him. Hit him a few times. Rinse. Repeat.

5. Go to the left, break the chest for the armor.

In the 4th Castle, you will get the ability to air dash. Once you have this go back to the 3rd castle (the one raised from the lake) and air dash to the right past the castle. You will come to an optional dungeon called The Sunken Palace. Beat the boss here to get the final armour.