To unlock the Shot Stopper achievement you must win a penalty shootout 3-0. This can be done in the Penalty Shootout mode. It doesn't need to be done in a real match. You can also unlock the 'Ace from the Spot' achievement while doing this one.
1. Menu Navigation:
On the home screen select 'Play' and then choose 'Penalty Shootout'.
2. Team Selection:
First, choose a strong team. To do this, tap on the blue bar on the bottom below the default teams (for me, the default team was L.A. Galaxy in the MLS league).
Second, choose the Barclay's PL (Premiere League) and then choose such as Arsenal, Chelsea, Manchester United, and the like. Ensure that the "Select Sides" slider bar is pointing towards the team you chose.
3. Opponent Selection:
Just as you did above, now select a weak opponent. Tap on the blue league bar below the right team. I went to MLS and selected Chivas USA. Start the match by selecting the check mark in the top right corner, then the soccer ball in the top right corner of the next page.
Being a Shot Stopper
You need to score 3 goals while your opponent scores 0 (either by your goal keeper blocking them or because they miss). (If you do this without missing a shot, you will also unlock the Ace from the Spot achievement).
Scoring a goal
Using touch controls, make a curved swipe to the upper left or upper right corner. Rinse and repeat. You should score almost every time, but if you miss a shot that's okay. You just need to score 3 goals before your opponent scores 1 goal. I missed a goal on 3rd attempt, then the opponent missed on his 3rd attempt, and when I scored on my 4th attempt to go up 3-0, the achievement popped.
Stopping a goal
As in real soccer, this is mostly lucky guessing. There are three options as a defender: dive left, dive right, or stay put. Watch as your opponent moves to shoot and then either guess if he's going left, going right, or going middle and then swipe your goal keeper to the left or right accordingly, or simply do nothing if you think the opponent is going to kick it in the middle of the net. Make your move immediately before the opponent is about to make contact with the ball. If you guess correctly you will likely block the shot. There is also a chance the opponent may miss the shot even if you guess incorrectly.
Once you have three goals, the achievement will unlock. If the other team scores before then, feel free to restart the match.