Pause the game and press Down, Left, Down, Left, Up, Left. Note: This code can only be used once.
At the main menu, press Up(2), Down(2), Left, Right.
Use a spear to break their armor first, then attack them with whatever desired. The gray bar is their armor amount. When fighting the second Giant Immortal, he will run away and start to regain health. Go to the statue he is beside, then push it on top of him to eventually defeat him.