In order to do this you must be on the Battle Table. The Ball thief power-up looks like a old-timey bandit mask. In order to get this trophy you need to use the Ball theif powerup while there are at least four balls of the opposing color on the table. Personally, I found the trophy to be a bit glitchy - I had done this at least 3 or 4 times before it finally popped. Keep trying and you should eventually get it.
In normal play the most balls either player gets at once is two. Therefore the best way to get this will be just after the opponent triggers the six-ball multiball. You can do this versus the AI or if you have two controllers in a two player game (much easier).
AI method: If you play on Battle mode, 10M target, with 25% penalty against easy AI you can effectively make the game last forever, simply by never letting your score get too high (let red balls drain on your side if you get too high). As long as you send a decent supply of balls to the AI side, it will not be able to score 10M (the 25% penalty is too much for it to overcome). Get a thief powerup, and wait for the AI to fill the multiball pistol (this may take a while). Once he does let the AI eject the balls, trying to keep as many in play as possible (the AI is very slow to eject balls, which is what makes this hard), as soon as at least 4 opponent balls are on table hit the powerup (square or triangle button).
2-controller method: (Recommended) Start a game with 10M target, 100% penalty, and both controllers. If at any point either score is getting too high then swap controllers and hit a ball over to that side's drain to clear the score and ensure you can play indefinitely. If you find it easier to fill the multiball on one side compared to the other (personally i find left side is easier) then make that the 2P side. First off play with the 1P controller and play until you get a ball thief powerup. If you get any other powerups use them to get rid of them. Once you have a thief power-up ready switch to the 2P controller. Now what you are going to want to do is fill the multiball pistol. You cannot do this until you first light up the 'shootout', by goign into the saloon and beating all the targets in one try. Once shootout has been completed (it will be full y lit in your table), the spot to lock the multiballs will be lit up (you will see a white bullet and arrow). It is in the center of the table just a bit off of the stampede ramp. If you are doing the left side the easiest thing to do is to launch a ball with only enough power to clear the first gate then use the upper left flipper to hit the target - if you miss the shot let the ball drain to get a chance to re-launch quickly.
Once all six 2P balls are loaded you are ready to go - pushing 'X' will eject the balls one at a time in the direction the pistol is facing. hold one hand over the X button on c2P controller, and the other over the square (or triangle) on the 1P controller. Once the pistol is facing up on the 1P side, quickly mash the X button to eject all 2P balls, and then push the ball thief to steal them. You may want to keep the balls in play for a bit to help to get the trophy.