*Credit to Virumor from YouTube for the video, and x MJ x on GameFAQs for the full game walkthrough below*
Alright, I'm going to give you a breakdown of the last boss fight, with a video to go along with it (fight starts at 1:44) so you can see exactly how he works. If you're reading this before the fight begins, take all the PA-1's to use for yourself from your allies (if they have any).
When the fight begins, you'll be battling some Sarcophagi (is that even a word?). Take them out, and when he finally appears, shoot the boss in the eye(s) (he's the three-eyed monster) a few times until he backs away and glows red.
The next phase has him shielded by the purple crystaline as well as sending Pey'J clones after you, so have Double H use his special attack so the clones get launched in the air, and use your slow motion attack (it's automatic if you didn't know by now, as long as enemies are tossed in the air) to take down the purple crystal shield piece by piece. There will also be Sarcophagi to contend with. You don't need to take down the entire shield, just enough to expose the boss. Your current objective is to hit the boss with one of the Pey'J clones using Double H's special + your slow motion attack.
The boss then takes away Double H, floods the room, and lifts your small platform up. At this point, he will be performing some very frequent teleporting around you. Stay on your toes, and hit him. When you connect, he'll immediately teleport to another spot around you. Sometimes he teleports above you--if he does that, just sidestep/dodge away until he's able to be hit again. If you manage to get hit and knocked down by him, dodge as soon as you get up otherwise you'll get hit again. Don't hesitate to eat Starkos or K Bups if you need to.
When you hit him enough, the room goes back to how it was, and the next phase consists of mostly Double H clones. One hit is all it takes to kill them, but they do use shields. I suggest you dodge their hammer, then strike, as they lower their shields to attack you.
When that phase is done, the boss tells you that you are losing control of your spirit. At this point some motion blur camera effects kick in, and all your controls are reversed. What I did, and suggest, is that you flip your 360 controller upside-down. You're not really going to need to move much, so you don't need to be very ambidexterous to handle that. First he'll begin by sending Sarcophagi at you, and after you take them out, he'll do the teleporting thing again. You simply need to repeat what you did before by dodging and striking.
After you land enough hits, the fight ends and you unlock the achievement almost immediately after. Feel free to grab a sandwich after the ending, as the credits are pretty long. There is a teaser after the credits, though, so if you want to watch it, don't exit the game during the credits.
Hope this was helpful, and any feedback is greatly appreciated. If you've got helpful tips, leave a comment and I'll add them to the solution.
If you'd like a full walkthrough of the game, check out x MJ x's walkthrough on GameFAQs here:http://www.gamefaqs.com/xbox360/609596-beyond-good-and-evil-...
It's got all the collectables, and can help you 100% the game, despite not being designed for achievements (since it predates the 360).