The following strategy is useful after unlocking the Nest of Evil portrait. There is a save point near the last room of the final area. Return to the surface and restock on potions, high potions, tonics, and high tonics. Afterwards, go back to the Nest of Evil and go to the last save point using the teleport stones. Before saving, unequip everything off of both Jonathan and Charlotte. The last Boss of this portrait is the infamous Doppleganger, who takes on the form of either Jonathan or Charlotte. The Doppleganger will have the same equipment, weapon, and skill of either character it mimics once you enter the room. After entering the room, re-equip your characters with the strongest equipment, weapon, skill, and team skill available. You will be able to take out the Doppleganger easily. After the fight, you will receive the last team skill on the skill list.
Take advantage of a glitch with one weapon that allows you to attack the Doppelganger without being hit back. First, equip the Illusion Fist weapon to Jonathan (obtained after completing Boss Rush Course 2 within five minutes). Give Charlotte the Encyclopedia weapon and Berserker spell. Don't equip any armor on either character. When the fight begins, backdash against the wall and call Charlotte to use Berserker, which doubles your now-unceasing attack. If you are fighting Jonathan, he won't attack you back. If you're fighting Charlotte, switch to the Kaiser Knuckles weapon for Jonathan and Charlotte won't hit you. This is a great strategy, especially when you want quick SP.