Once you are in the area, you need to enter the water so the Colossus sees you, his eyes go red and the music intensifies.
When he goes to attack, move out of the way and then turn back and grab on his tail which comes out the water.
Whilst grabbing, go up his back and attack each vital point, they are behind the electric spikes on his back.
The last weak spot is on his head, he goes in and out the water quite quickly here, so you need to be quick.