This can be done on classic or battle table (I did it on battle, but ive read elsewhere that it works on classic). Which table you want to do it on really depends on your skill with keeping the pinball alive. On classic table it is rather easier and faster to fill the gun, however you have to worry about the game ending because you run out of balls. On battle mode, it is much more laborious to fill and fire the gun, but you can set the settings so that one game can be of indefinite length. The trophy will award mid-game as soon as you meet the requirements.
Classic Table Method: Unless you are really good at loading the pistol, you probably can't get this within a 10 minute time limit game, so you will want to play on the 10-ball limit. If you still are having trouble running out of balls, slog it out in battle mode (see below). The gun in the single player table is loaded via a slot that is located right above the left flipper. There are two basic ways to load the pistol, either by hitting the Ace/Queen ramps, or by shooting it directly into there. On the drain back ramp after hitting the queen ramp (or exiting the saloon after hitting the Ace ramp) there is a little toggle which diverts every second ball that passes into the pistol. Eg you hit the Queen ramp once and this toggle goes into place, now the next time you hit the Queen ramp the ball will load into the pistol (and you get a free ball in the launcher). Ditto for the Ace ramp/saloon exit and note you can mix and match. In order to directly hit the ball into the loading hole, you pretty much need the upper left flipper to be held up. The most reliable way to load it is to catch the ball on the right flipper, and then with the left flipper also held up shoot it into the loading hole. Be careful with this shot as with the left flipper up there is a good chance to drain the ball if you are off target. You can also hit the loading target off the left flipper but this is a pretty tough shot (has to be a motion shot you cant do this with a caught ball), and you should really be aiming for the Q ramp if you have a play on the left side. Once you load 6 balls into the pistol, mash the x button to eject them. Do this three times and the trophy is yours.
Battle Table Method: You can do this versus AI or with a second controller. If you do AI use easy AI, to a 10M score limit and 25% or 50% penalty, and manage the score for indefinite game length as detailed in my other solutions for . In battle mode the person who puts in the sixth ball gets to 'fire' the pistol so on one hand AI can help you load the pistol, on other hand the AI can 'steal' the loaded pistol. Overall, I would recommend using a second controller if possible. In that case use a 10M score game and 50% penalty so that you can work on progress towards the hot potato achievement at the same time (hot potato power does not seem to spawn on 100% penalty game). If your score is getting close to 10M, grab the 2P controller and shoot a few balls over to the other side to drop the score down low enough.
Whichever way you go for (AI or second controller) you want to pick the side of the table that you find easiest to load the gun (personally I prefer left). The thing that makes firing the gun a hassle on the battle table is that you cannot load the pistol until you complete a 'shootout' round, by hitting the ball into the saloon and knocking down all targets without letting the ball drain out of the saloon. once this has been achieved, the drop hole for loading the gun will light up (it is just above the stampede ramp). Simply knock the ball into this six times and fire the gun. On the left side of table the easiest shot is to launch the ball softly enough to have it run past the upper left flipper and then shoot it in directly (you can also make the shot from the lower left flipper but its harder and it usually goes up the stampede ramp). Once you fire the gun, the pistol loading light will be off until you complete another shootout (dont worry about if the shootout light is already lit, you will still light the pistol loading ramp just by repeating a successful shootout). You will need to repeat this cycle (complete a shootout, then load and fire the gun) three times to get the trophy.