Amazing guide! Well done for putting this up!
I have something I'd like to add that I've noticed from playing this game. Well it's actually a couple of things, one isn't of too much consequence, but the other is a GREAT help for an achievement.
First thing, I've found the Deadly Nadder has an attack with a longer range than the standard Y attack. The standard X attack has a slightly longer range, I noticed that as the opponents were just out of range for the Y attack I could just press X once without moving and the attack would connect. I wouldn't recommend switching just to the X attack though as the time between doing the attacks is longer than the Y attacks. But if your opponent is using the hanging back trick to escape your swing, try hitting them with X first, might help out.
Secondly, there is a way to make the Memory Torch on Expert insanely easy. All you need is quick reflexes and a phone or notepad open. Basically position your hand on the controller so your finger is over the pause button for an instant press. The second the first torch flashes PAUSE the game, write down the button that corresponds to the torch and then unpause. You should have one second or so before the next torch flashes. When it does, repeat. Then when it says "Your Turn", pause again and look at your note to get the sequence firmly in your head. Unpause it and hammer in the sequence. Perfect every time.
I will also say that I too have experienced the time issue. On my first playthrough I had unlocked all the achievements, including all the Arcade mode ones, the only two I had left were finish the story with Hiccup, and the 20 hours one. When the 100 Arcade wins popped I saved the game to discover my total playtime was 16 hours and 19 minutes. I kept my xbox on with a wired controller plugged in while having Astrid stand outside the House in Story Mode for 4 hours. The guide is very efficient, perhaps too efficient, either that or this game is far too simple to beat in over 20 hours.