The Custom solution didn't work for me. I popped 6 on a custom board many times, and it never worked.Here is how I got
I played an expert board and randomly clicked on squares across the board. When I clicked a bomb, I quickly started a new game and continued. It took around 5 minutes for me to randomly click away when the achievement finally popped. I didn't actually see a six, the achievement popped AFTER I lost the game. Other people have now reported the same thing (finally popping without a 6).
THIS WAS BASED ON LUCK. JUST CLICK AWAY.
I will note that I did get a 6 a few times in the daily challenges and I NEVER GOT the achievement. It was only in the expert board randomly clicking away!