the most difficult achievements
I got killer king, Castle Victory, The one where you win with less then 1 sec, the one without losing pieces.
castle checkmate isn't that hard, you just get the king trap'd on a open file, save castle, also I am rated 900 in blitz, I went to 10 minute chess and since I was a low elo in it, it was easy to get the achievements there.
So I recommend to pick the time setting you have the lowest elo on, as it makes it a lot easier.
I like this list. It gives me goals. And with this post I should get the 1st Word achievement, perhaps one of the easiest achievements?
That and first puzzle and first game.
Easiest of them might be "In the nick of time" , just get someone into checkmate in 1, then wait till your under 1 second to checkmate them.
I got all checkmate achievements, including killer king, scroll up and watch the game that's how I did it.