To defeat the large Boss on the mechanical tower stage, continuously hold R to block, and dodge all his attacks by pressing A while blocking. Control the direction of your dodge by moving the Analog-stick. Note: Dodging to the left or right is best. Blocking his hits will not work, as you will die after four hits. He has no armor on the back of his legs; when you get behind him, hit his legs and prepare to dodge again. After a few times of doing this, he will fall to his knees. You will be able to jump on his back by pressing A. Strike him in the head with your sword five to eight times. He will then throw you off if you do not jump off in time. Repeat this three or four and he will die. Roll under his legs (it is a lot easier than trying to go around) and slash at the back of his ankles. When he drops to one knee, press A to climb up his back.
About halfway up his back, slow down time and begin slashing at him. You should be able to kill him before he can throw you off his back. Also, when he dies he gives you a sand tank, so it is like earning it back.