This one is a little tougher than the other complete stage # achievements, even on easy. I had some difficulty getting past Pai on Stage 4 because of her countering. Here are some tips that helped me get past Pai and the other fighters.
-Use a character you are comfortable with. You can't choose another fighter when you lose / get to the Continue screen. Akira and Lau are good picks if your new to the game.
-However you can lose and continue to try that stage again. So keep trying!
-Guard (the X button). But don't guard too long or the AI will throw/grapple you!
-Rising kick (press Forward + B when you lying on the ground to kick as you get up). This one helped me alot.
-Heads-up attack (press Up + A to attack them when they are downed). This was super helpful against Pai.
-For settings I used 2 Rounds and 15 Seconds. After losing quite a bit to Pai, I dropped the difficulty to Easy. She won't counter as much.
-Doesn't matter how you win, just win the Stage to continue to the next. (Time Out, Ring out, Victory).
When you get to stage 5, win the stage. The Achievement will unlock on the win / grade screen.