1) To make obtaining this and all other achievements faster go to options and turn speed play on
2) Custom play -> Rainbow -> Two players -> select location (no need to change default location)
3) Select human for player 2
4) With player 1 (you) roll dices once and select square with most points.
5) With player 2 roll dices once and select square with “-“ mark or lowest points.
6) Continue until you finish the game and you can start new one right away.
I suggest that you complete adeventure mode first and then classic, rainbow, duplicate and battle modes until you have unlocked Classic master, Rainbow master, Duplicate master and Battle master. Then check your stats for Victories (Extras -> Statistics). You should have around 40 victories. Then if you start with Rainbow mode, you should unlock Rainbow sensei at around 130 victories. If you jump from mode to mode, you can't keep track how many victories is needed before achievement unlocks.
Starting from Custom Play menu it takes about 1min 50sec until you are back in Custom Play menu and ready for another game. I think you can make it in 1min 30sec if you really focus and just select point squares for P1 no matter what points it has. So roughly it takes 150 - 180 minutes to complete.
As Uranium Deposit mentioned make sure you don't select "Duplicate Raimbow" as it won't count.