Here are some tips:
- Forget breaking his armor. There's no time for that. Get him to swing at you, then run about halfway up his sword and crouch down. When he stands back up, keep crouching to keep your balance. If you slip and grab the edge, just pull yourself back up and crouch again in the middle of the sword. You'll then need to jump towards his arm JUST BEFORE he shakes his sword. His arm will move down just as you're jumping up, and you will be able to grab onto his arm.
- Once that's done, go for the stomach vital first. You can jump right from his elbow to this stomach with little trouble. Stabbing the stomach is annoying, but you can always get a full stab in when he stops walking and starts to shake side-to-side. Just get your stab in, then let go and rest on the stomach platform while he's walking. When he stops to shake again, grab on and get in another stab. Rinse and repeat.
- Once his stomach vital is dead, jump back to his right arm. From there, climb up to the head.
- The head can be frustrating, but here's a little trick. If you press Square like a microsecond after you first grab on with R1, the game will usually let Wander get in a full stab no matter how much the colossus shakes. Also, try to position yourself on the back of his head so you're just barely stabbing the vital. That's the most stable position on #3's head.
- If you still have 40 seconds or more left after his head vital is killed, you have time to finish it. Jump from his back down to his left elbow. Try to land as close to the vital as you can. You won't have to worry about time when stabbing this vital, but you do have to worry about grip. Don't miss any stabbing opportunities here or else you'll run out of stamina and fall. Three full-ish stabs should do it.
CONGRATULATIONS! You've beaten HTA3!
Now HTA15, on the other hand, that will make you suicidal.