Right after you rip Medusa's head off in the beginning, you have to freeze all of the minotaurs and destroy them when they're stone. Since you have unlimited magic during this, instead of freezing them, simply keep using Poseidon's Rage and the Minotaurs will keep respawing leaving you with as many kills as you want and as many red orbs until you're bored. Also, make sure to save up a LOT of additional red orbs to be able to afford all the other powerups later on in the game.