If you step outside of the Shrine and go behind it, on the left you'll see a land bridge
above the area where the 2nd colossus was located. When you cross that bridge and use your
sword to point the way, you should see a cavern in the mountain. Go into that cavern and
it'll come to the scene that lets you know that you're in the right spot. Notice that
there is a huge platform in the middle of the water and there is a ramp on the left side.
Swim to the ramp and just go up it, you'll have to jump onto a chunk of rock, shimmy
around it so the platform is behind you, and jump onto that. Then get ready to fight! woot
woot.