I've never hunted after achievements, but still cool stuff.
The rarest achievement in chess.com!?

Some achievements are really easy, but I think checkmate with all the pieces on the board is the hardest

I have it too, but it is mostly luck
The one for 10,000 puzzles would take forever (I dont have that one)

I have it too, but it is mostly luck
The one for 10,000 puzzles would take forever (I dont have that one)
yh frfr but its not hard its js time consuming

According to me, the one that is hard is get is the discovery checkmate with king or checkmate by castling

what abt the the achievement for winning with only 0.1 seconds left?
My records show that this was awarded on 11 February 2018. I think the awards were new then. I'm certain I've done this dozens of times as I've played substantially more than 100,000 blitz and bullet games, although only 70,000 on chessdotcom.

and I forgot to tell you that it's a secret achievement and you can only win it by deliver checkmate with all the pieces still on the board. (including your opponent's.)
Like full house, castle victory, all the piece mates and nick of time