Instructions
Save your game as soon as you get to the Temple of the Oracle after making it through the Path of Hades. The battle with Ares --- the God of War --- is a tough one, so it's a good idea to save so that you don't have to start over from the last save point in Hades when you die.
Go into the Temple of the Oracle and defeat the hordes of enemies that start spawning. Avoid using your Rage of the Gods spell --- you'll need it for the Ares battle. When you've defeated the enemies, proceed through the temple, following the same path you used the first time you were there. Open the chests near the save point to replenish your health and your magical energy, then save the game.
Exit the hallway to see a cutscene and initiate the final battle of the game. Use Rage of the Gods early in the battle to give it time to replenish so that you can use it again for the second stage of the battle.
Cast Army of Hades as often as you can. This spell draws out a lot of green and red orbs from Ares so that you can keep your life and magic meters full. Once you do enough damage, a button appears over Ares's head signaling that you should do a kill sequence --- although he won't actually be killed yet, you must do this to advance to the next stage of the battle. Press the button and follow the sequence of buttons to see a cutscene.
Fight your shadow clones to protect your family. Hordes and hordes of Kratos clones appear, making it impossible to prevent all of the attacks on your family. If their life bar drops too much, go up to them and hold "Circle" to transfer some of your health to them.
Use Rage of the Gods when its meter is full, and cast Army of Hades as often as you can to keep the Kratos clones at bay --- Rage of the Gods gives you infinite magical energy as long as it is in play. There is no meter or countdown for this portion of the battle, so keep fighting until the next cutscene occurs.
Fight defensively for the last stage of the battle. You can't block some of Ares's attacks during this round, so your best bet is to run around the arena when he starts launching attacks. Stay relatively close to him, because he gives out a war cry after most attacks --- take this opportunity to attack him. Occasionally, if you do enough damage when attacking, a button sequence will appear on-screen. Press your controller buttons as instructed to deal even more damage.
Continue attacking Ares until the battle is over. Move forward from the battle arena once you regain control of Kratos, and walk up the stairs. Go through the glowing doors to Mt. Olympus, then save your game. Sit on the throne to watch the final scene and ending credits.