Try as you might, if you search the fortress for another body to possess, you will not find it. So all that is left to do is head to the final dungeon.
Reaching the entrance to dungeon 6 isn't terribly difficult. Begin to move through the fortress as though you are on your way to the first dungeon. Once you are high enough in the fortress and see a Mummy in front of the staircase, you will detour and start moving towards the second dungeon, as if you were going to fight the Fighter again.
After you climb up the ladder to the right of where the Fighter awaits, jump off to the right, and trigger the collapse of the false floor that lies just to the right of another pit. Fall down the hole in the floor and continue to push left.
As you fall, you will eventually reach an alcove in the wall, where the sixth dungeon entrance is. Of course a Bouncer is waiting for you, so you can attack it or ignore it if you like. Destroy the blue blocks surrounding the dungeon entrance and hit it to head inside. Make sure you have at least four keys before you enter.
The bad news is, this is the longest dungeon of them all. The good news is, you won't have to travel back through it to return to the fortress.
More good news is that as soon as you arrive, the next body for you to possess will be waiting right there for you to attack. Defeat the Holy Soldier as you would any difficult enemy, crouching low, getting inside quickly, and attacking frequently. As soon as you knock it down, you will take possession of the final body of the game.
There is good and bad news once again. The good news is that when you possess the Holy Soldier, you will start out with a full 12 unit health meter. The bad news is that there are no statues from which you can obtain energy items until you reach the very top floor with the boss, so you will need to hang on to as much health as you possibly can all the way through the dungeon.
Traveling through the dungeon shouldn't be any more difficult than the rest except for the length. The most dangerous enemy are the mudman who will spit at you. Crouch low whenever you encounter them. You will need to use every trick you learned, but none are more useful than walking backward below the bats. There are a number of them in low ceiling areas, and you won't get many good opportunities to attack them, so if you walk backwards below them, you will simply guard against their attacks while you proceed.
The final boss of the game, the Prince of Hell awaits at the top of the dungeon. When you arrive at the top, you will see statues, most of which contain swords. Many of the statues beyond the doors contain energy, so collect every single one. When you reach the room where the prince appears, there is no safe spot from his attack; he can damage you anywhere you are standing. The best strategy is to rush him and attack him despite his current behavior. If you jump up to him and attack him quickly, you will find that one sword thrust is all it takes to do him in.
Once he is defeated, you can continue on to the room to the left in the very back. Jump up to the steps, and the Princess of Light will appear, rescued from her prison. Congratulations.