The final battle against Ratash proves to be pretty easy, as long as you keep using the Stone Armor. Make sure on your way to him (killing tons of enemies during the sand storm) you refuse to use any energy slots. The very nice thing about this boss battle, is that even if you beat it and had not obtained the achievement, you can still continue from the last checkpoint at the main menu. This will allow you try again. You will be able to fight him again as much as you want as long as you don't start a new game. You do not get any checkpoints during the fight, only at the start. Here is a video on how you should beat him.