This option is unlocked at the 'Main' menu by pressing Up, Up, Down, Down, Left, Right.

Begin by breaking their amour using a spear and then attack them with whatever weapon you have (the gray bar tells you what armour amount they have left). When you face the second Giant Immortal follow him when he tries to escape to regain health and he will lead you to a statue. Push the statue on top of him to defeat him.

During gameplay pause the game and press Down, Left, Down, Left, Up, Left. This code can only be used once so use it wisely.