You can set the board size to anything using the custom game, but that's irrelevant. What this achievement requires is winning one of each of the preset game types - easy, medium (16 x 16) and expert (30 x 16).
What won't work is creating an easy custom game with 16 x 16 and 30 x 16 board types - I tried it. Don't bother.
Even expert is not too bad, but if you're really desperate, it's helpful to know that you can retry a board after you lose and it still counts. Why this is helpful is that when you lose you get a brief snap of all the bombs on the field as they explode. So if you take a photo or screenshot at the right moment, you can retry on the same board and you'll have a readymade guide.
As I said, though, expert (30 x 16 with 99 bombs) is nowhere near as bad as the hardest levels on the WP7 minesweeper (there were only a handful of 5 squares and nothing higher when I played it). So you may well not need any help to get it.
As soon as you win the third board type, the achievement will pop.