The description is misleading it should read;
"Play your first 75-99 point word"
Basically just make a word that is 75 points or more but not exceeding 99. I personally saved up tiles to make the word "QUEENS" that went across a double letter score where my "Q" was and a triple word score on my "S" for a total of 78 points.
By "saved up tiles" I mean I held on to the letters I would need (Q,U,E,N,S) and built the gameboard up using mostly the 2nd player while swapping out tiles and passing turns until the correct player had the letters to earn the achievement and the board was set so that an additional "E" was in the proper place to set the tiles on the appropriate point multiplier squares.
This can be done in any play mode.
If you vote negatively please let me know why.