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.

Time Attack Strategy
Avion is a frustrating time attack. He never seems to calm his wings down enough for you to eliminate the vitals on the tips and the first minute and a half is spent swimming over to one of the ledges. Thus as a forewarning, unless you're very good or very lucky, it will take several tries to make your time. When you start the battle swim just to the right of straight forward and grab the center platform. You should be staring straight at Avion. Hit him with an arrow and grab on.

If you're in hard mode, you'll want to take out his tail first. You do have to make an extra trip down his tail, but it's worth it because you won't be concerned about your stamina on the last wingtip and can use every bit of it, which is essential because you'll be using every last second to beat this time attack. Quickly eliminate the tail vital and head back up. While he's gliding dead ahead, go ahead and run, don't crawl, you'll save precious seconds and recover important stamina. When you get back up to his body pick a wingtip. Wait until he starts gliding straight and then quickly run across and get to his vital. If Avion is still gliding, go ahead and kneel on the not-furry section of his wing for just a moment to gather your stamina (you'll want it to be full by the time you grab the first wingtip).

Each wing tip will take two fully charged stabs to eliminate, and you must be quick about it. After each strike, Avion will do a barrel roll and take a large chunk of your strength away. If you fall off, you'll have no hope of making time. Once he settles down quickly strike again and hold on for another barrel roll. If you're lucky he'll glide straight forward again, just in time because you've probably run out of stamina. Scamper across and recover your strength by standing near his mid section. The second wingtip is somewhat easier because you don't have to worry about saving stamina to hold on for the second barrel roll, which is why you went for the tail first and the wings last. Repeat the process for the first wing and if you're good and lucky you should kill Avion with just half a minute to spare.