Shadow of the Colossus/Avion - Colossus 5

Reaching Avion
Go east again, like you did to reach Phaedra. When you get to the mountains, go around them via the misty ledge to the north. Keep going until you reach a lake.

This battle takes place over this body of water, which is nice considering that falling off the colossus is in the nature of the battle. To reach the inner area of the lake, swim through the crack in the outer wall, then jump-climb up the ladder and approach the balcony. A cutscene will play showing the fifth colossi, Avion the Bird, entering the area.

Defeating Avion
Drop down into the lake and find the series of blocks near the top of the lake (close to Avion's perch). These are perfect for doing what you've got to do. Climb on, equip your bow, and shoot an arrow at Avion. He'll screech and start flying around the lake.

For him to notice you, fire another arrow at him. A second shriek means that you've hit the Colossus. Avion will come around and swoop down to attack. Getting on him can be tricky. Run and jump towards the base of a wing when he's close enough in his decent. This area is furry so grab on and climb atop his back.

Avion has three magical weakpoints for you to stab - one on his tail, the other two on each of his wings. Regardless of whether you're on Hard Mode or not, you have to stab all three to win. The wing weakpoints are considerably harder to reach and stab, since your foe has to flap them in order to stay aloft. Also, Avion will do a barrel roll when you do land a hit, which will drain a good portion of your stamina. You have enough to finish off one wing and then make it back to the relatively stable body.

The tail is surprisingly easy to stab, so simply run down the length of the tail until you reach the end and the weakpoint. Avion does banking turns in the sky, so you will need to hold on.