Majin and the Forsaken Kingdom

***SPOILER ALERT***

This achievement is story related and so can't be missed. If you don't want to know any details of the final fight then stop reading now. You have been warned!

If anyone has some good quality Youtube links for this it would be really helpful. Best I could find were from the PS3 and missed some vital aspects :(

The final battle is really two fights each with multiple stages split across two rooms. Note that the game will checkpoint between each stage so don't get too frustrated if you die.

Room 1:
The king will cause the Majin to become cursed with darkness. So you can only control Tepeu for this fight. Ignore the Majin, darkness soldiers and all random pickups (the last bit is very important - the bombs and conductors have no purpose in this room and are just to distract). If any enemies are becoming too annoying then either try and knock them out using the purple eyes on the floor or just run away to try and gain some ground on them.
Stage 1 - There is a valve on one wall which you must turn by repeatedly pressing B. Keep doing this until you get a mini cut-scene.
Stage 2 - The valve will have brought a platform with a crystal block down into range of Tepeu's jumps. Jump onto this and destroy the small masks on each side of the block. The Majin will try to electocute you from below. Run around to try and get him away from the centre of the arena. He is much slower than you. Once all masks are destroyed time for another mini cut-scene.
Stage 3 - The crystal block will have fallen to the ground. Run up to it and you will be prompted to press B to shatter it. Rapidly press B to do this. You will need to do this twice. Try and just ignore the enemies attacking you. It is not worth killing them as they will simply respawn. You will be rewarded with another cut-scene and out will jump Toci.
Stage 4 - The Majin will now remember why he is here but the darkness mask will prevent him from fighting alongside you. Jump and attack the mask several times pressing B when prompted to break it.

Room 2:
You now have control of the Majin again. The king will now run through to a second room threatening to bring the 'Creator of All Darkness'. Scary, eh?
Stage 1 - Attack the head until prompted to press B. Nothing else to it. You will be rewarded by the boss getting much harder...
Stage 2 - The boss will now pull himself through the opening and there will be two arms on either side. The lower arms will cause a shield to pop up around the boss making him invincible and the upper arms will send balls of light at the Majin with a similar effect to the monkey and ghost enemies earlier in the game. Send the Majin to attack each of the lower arms. He will use his Purification ability to seal up the doorways they are coming through. Ignore the upper arms - you don't have time to seal all four doors - just help out the majin by attacking the balls of light whenever he is in trouble. Now attack the head again. Eventually you will be prompted to press B and another cut-scene will occur.
Stage 3 - The final stage!! Before the boss has time to recover, back off and heal - it is the last chance you will get. This time the boss has all 4 arms free. He will shoot beams of darkness from his hands at the Majin. You must quickly go around and attack each hand. Once this is done attack the two arms that have fallen to the floor, press B and enjoy a job well done.