Zelda: A Link To The Past: help needed with the 5th crystal palace

Hello, Thanks for taking the time to respond to this question. I'm stuck in the 5th crystal palace. I have the big key, the compass, the map, and the armour, BUT I can't figure out how to get to the main boss. Please help.

You mean the Ice Palace, right? It's hard to explain, because of the one puzzle (and I haven't played it in a while). So let me quote the walkthrough from NoA official site. This part tells you how to get there from the chest with the Blue Mail (the armor you just got) (For future reference our main site also has walkthroughs of all the games):

Originally Posted by walkthrough

From the room with the Big Treasure Chest on B5, go through the door on the right and enter the Big Key Door. Enter the next locked door to appear on B6. Go down through the door and enter the locked door on the right. Hit the crystal switch to make it turn blue, go back through the door on the left, and go up through two doors to appear back on B5. Go down through the door, enter the locked door on the right, and lift the jars in the top-right corner to reveal a floor switch.
Step on the floor switch, go down through the door at the bottom of the screen, and enter the next door. Go up through the door to appear on B4, enter the locked door and lift the jars at the bottom of the room to reveal a floor switch and step on it. Enter the door on the left that opened and drop into the hole to appear on B5. Go right through the door and push the lower-left block into the hole and drop into the hole to appear on B6.
Lift the jar to reveal a floor switch, then grab and push the block onto the switch to open the bottom door. Enter the bottom door, go to the left side of the room and grab and pull either one of the statues up. Use the Hammer on the face switch, Lift the large block, and drop down the hole to appear in the room with the Boss, Kholdstare.

Official NES/SNES Zelda Gameplay Help