I had a guide 1/2 typed when yours appeared
Most of the achievements are self explanatory.
I was going to order the achivments into sections as such as Tournaments, Challenges, Arcade, as I personally find the best guides bunch all associated achievements together to limit scrolling up and down the guide every few seconds.
One major thing I would change is your combat tactics, you got the right dragon to use and you got the right attack button but quite why your advising people not to use
combo I'll never know. This combo is the game breaker the only combo you will ever need as it stuns and you can keep them permanently stunned.
Soon as the fight starts , spam the combo, if you get a stun quickly dash back, now ideally you want to be 3 1/4 dragon lengths away and use the combo again on your stunned opponent. The 1st shot is a long range tail whip this will free him from the stun, the 2nd attack will miss as your too far away, but this is what we want, the 3rd hit will re-stun him.
Rinse repeat and your will not ever lose a fight as long as you get your distances right. Worse case scenario if your a little close you get a full 3 hit combo and decimate his health anyhow.
Also I would recommend that the 1st play-through be done as Hiccup as this will be your grind play-through anyhow.
Reason Astrid is better is as you say she starts with the Nadder and this is great for your second play-through as you need not bother at all with any other dragons training. I cleared the whole game 2nd time by getting her starting Nadder to level 15 by training enough to unlock the combo mentioned above and by blitzing through the easy and normal challenges.
Took less than a 3 hours 2nd time through and that was with messing around to get the 20 hour achievements. Both campaigns I retired undefeated without a single KO.