At the top of the temple, open the door to be thrown into Kratos' memories. Go the only path you can until a cutscene plays. At the end, you'll be thrown into a boss fight with Callisto.
BOSS FIGHT: CALLISTO
This boss fight can be a little annoying because you don't have much room to maneuver. Callisto's main attacks involve pounding the ground in front of you, swiping the ground in front of it, and charging at you. If it catches you while charging, it will toss you into the wall and sometimes follow up by pinning you against the wall. Alternate between L1 and R1 to get loose, then hit the correct QTE button to break free. Note that you can get some free hits after first getting loose, so either use light or heavy attacks to smack Callisto some.
Jumping is a good way to avoid the sweeping attack, and as long as you keep moving, it should never really grab you when charging. The best opportunity to attack is between its attacks, but the biggest chance is when it charges and misses, where it will hit the wall and be stunned for a second or two. When it's finally knocked down, go up to it and press Circle to finish the fight.
EYE OF ATLANTIS
Once you regain control, make sure you pick up Callisto's Armlet next to you, although you can't use it until you've beaten the game once. Go into the next room, where you'll be trapped, so go up and grab the Eye of Atlantis, which is your first magic spell. Press the right directional button to use it, which sends out a bolt of lighting straight ahead of you. Normally, using it will use up some of your magic meter (blue bar), but you have infinite magic in this room.
With your newly found power and the convenient infinite magic, use the Eye of Atlantis to kill all of the enemies in the room. Note that if you manage to not get hit, this is a great chance to get the I'd Hit That trophy for a 200-hit combo. With the gates now open, go down the right path to find a Gorgon Eye and Phoenix Feather. Collecting 5 Phoenix Feathers will increase your magic meter.
Go back and take the left path this time. When you get to the balcony, Scylla will appear once again, grab you and drag you away. Note that when you land, there is a quick QTE, so pay attention. From where you land, follow the path around, grabbing the Phoenix Feather from the chest, then save at the Save Point (grab the health or magic if you need either). You're now in Methana Volcano.
