When you first get Medusa's Gaze and are asked to kill all the minotaurs, refuse the tutorial and instead cast Poisiden's Rage over and over. Kratos will have infinite magic in this "training" room since the game expects you to learn the Medusa gaze attack. With the respawning enemies, magic, and Poseidon Rage, you can rack up huge comboes and gain enough XP to max out Kratos early.

Remember to stock 25 to 30 thousand more XP than you need at that time so you can max out your weapons, magics, and skills later on.