After the scene, go up the stairs and into the temple. Follow the path up, defeating the monsters along the way, then jump out of the temple to the area where you saved the Oracle. Find the save point, save, upgrade anything you need to (with emphasis on the Army of Hades spell if you haven't upgraded that yet), and go into the entryway and through the hall to find Ares.
Part I: Battle of the Giants
Kratos finally gets his hands on the power of Pandora's Box, and uses it to grow to the size of Ares. Use Rage of the Gods as soon as the battle starts, and right afterward the Army of Hades. You're going to need magic later on, but the attack takes enough magic (and health) out of Ares that the spell ends up costing not too much. As soon as you see the prompt, press and begin the QTE to knock even more health/magic out of him. If you haven't leveled Army of Hades enough, you'll have to stick to a defensive position. His attacks are much stronger than yours, so block and dodge often. Finish the battle for a scene.
Part II: Clones
Ares sucks Kratos into a portal he creates, giving Kratos no choice but to face the challenge Ares has in store for him. In this segment of the battle, you'll have to face numerous clones of Kratos, ready to murder his wife and daughter as Kratos had done in the past. Defeat them to keep his family safe. If they run low on health, press to transfer your health to them. If you're having too much trouble here, use Rage of the Gods. Defeating the clones should give you even more magic, so use Army of Hades and Poseidon's Rage whenever you get the chance.
Part III: Final Battle
Once every clone is defeated and the scene is over, you'll have to go against Ares in the toughest segment of the fight. Ares takes away Kratos' weapons and magic, so you'll have to be extra cautious here. Fortunately, Kratos is able to find a sword which fits his size nearby, and the battle begins. Utilize Kratos' dodge frequently here. Ares has a multitude of dangerous attacks, including one in which he sinks into the ground and reappears right in front of you. Occasionally, you'll get into a power struggle with the god; in cases like these, press rapidly to avoid taking some heavy damage and instead give yourself a boost in the fight. Attack enough so that the bar on Ares' side is almost fully depleted, and the bar on Kratos' side is almost full. When that's done, Kratos will finally get his revenge on the God of War.
Congratulations, you've beaten the game! Now enjoy the bloody ending scene; you've earned it!