I'd like to see you make a FoP capable of that with only 6 levels.
I'll take that as a challenge. How debatable can the tricks get?
Sure. CHALLENGE TIME~!
Be as cheesy as you like and we can simply measure how awesome you were by subtracting how much cheese you used.
You're on.
1. Be an elf or whatever. Just make sure you can take Otherworldly.
2. Take Otherworldly.
3. Take Educated or some other feat that permanently gives you Knowledge (Arcana) as a class skill. Stupid FoP requires it but doesn't have it as a class skill.
4. Level to 3rd level. Use Craft skill cheese to make enough money to pay for a Ritual of Alignment. When you have enough money to buy a scroll of Flesh to Salt, you can speed this up by a lot.
5. When you're lvl 3 and have enough money, perform a Ritual of Alignment to gain the Evil subtype. The works.
6. Regain your XP to get to level 3 so you can enter FoP.
When you reach FoP 3 and ECL 6,
rebuild one level of whatever base class you chose to another FoP level.
7. At FoP 4, you gain the ability to possess creatures.
8. Profit.
If you want to be able to actually control the creature as well, that's possible too. Just rebuild the other two levels of your base class as well and end up as a FoP 6 without base classes.
Alternatively, you can use a certain debatable trick (now, unfortunately, lost to the WotC boards) to get around the skill cap and get into FoP a level earlier. Same difference, really.