Map: Sealed Grounds
Recommended Element: Light
Heart Container: Fi
Heart Piece: Link
Skulltula: Defeat 1,000 enemies
Start off by cpturing the nearest keeps and outposts. After a short while, some of your allies will turn on you. You have to hunt them down. While you're doing that, more allied groups will turn traitor and to make things worse, the enemies will receive more reinforcements from inside the temple. Reinforce your nearby bases while you're hunting down the Turncoat Leader.
If you still didn't find the leader, the other traitors will attack your allied base. Head there and defeat them to lower the threat in your main camp. Fi will also narrow down the search area. If you still haven't located the turncoat leader, he will lead an attack to your camp himself. You can finish him off there. If you managed to find the Turncoat Leader (he's in the main allied base when I found him but the location may vary per game) Defeating the Turncoat Leader will force the other Turncoat soldiers and their forces to withdraw so if you're after the Materials, it's better to hunt them down instead and save the leader for last.
After defeating the Turncoat Leader, Ghirahim will summon a large boss called The Imprisoned. Head there and take it out first. To reveal the weak point of this monstrosity, you have to attack its wobbly toes. Destroying all of them will upset the monster's balance and will make him fall down. Attack it while its down until you reduce its Weak Point Gauge to at least half. If you haven't captured the West Cannon Keep yet, head there first and capture it. This will allow you to use an emplacement in that Keep against The Imprisoned. When the Imprisoned reaches the West Cannon Keep, it will float and charge up for a powerful attack. Head to the Keep and use the catapult there to stop The Imprisoned from firing its blast.
The boss will make his way to the East Cannon Keep this time. You can still intercept the boss and destroy its regenerating toes but be careful of the powerful shockwaves released by his every step. You can attack then time your evasion just in time before its foot hits the ground. If you still haven't captured the East Cannon Keep, head there and clear it out. Capturing the base will also reveal a chest containing a Sealed Weapon. When the boss reaches the base, wait until the catapult becomes usable then fire the final shot that will defeat the boss.
You'll now have to hunt Ghirahim. His forces will also march towards your main base so it will be a good idea to stay near your base for the meantime and take out the enemy forces. Once there are no more enemies attacking your base, head to the Temple and engage Ghirahim.
He has a Moblin and a Stalmaster with him so if you want to get their Materials, destroy them both first. Ghirahim isn't that hard to deal with. Wait for him to attack, move to the side while you have your shields up then start your combo. He will reveal his Weak Point Gauge when summoning a magic circle. If you're able to defeat the Imprisoned without too much trouble, defeating Ghirahim should be easier.
After completing this mission, you'll unlock Zelda as a playable character.