The following trick is helpful if you have already unlocked the Nest of Evil portrait.
When you get to the last area, there is a save point near the last room. Go back to the
surface and restock on potions, high potions, tonics, and high tonics. Then, go back to
Nest of Evil and go to the last save point using the teleport stones. Before saving,
unequip both Jonathan and Charlotte of everything. The last Boss of this portrait is the
infamous Doppleganger. The Doppleganger will take on the form of either Jonathan or
Charlotte. The Doppleganger will have the same equipment, weapon, and skill of either
character once you enter the room. After entering the room, quickly re-equip your
characters with the strongest equipment, weapon, skill, and team skill for them. You will
be able to take out the Doppleganger very quickly. After the fight, you will receive the
last team skill on the skill list in the next room.