First and foremost, you must equip an Assit Gem to one of your gem units. You do this by going under "customize" and selecting gem unit. Then click R1 and look at the "Assist Gems" category. What you'll be searching for is the free auto-block assist gem as it's the easiest to activate. You can't have any other gems, so simply remove all of your gems from sample 1 using triangle and then put on the assist gem. It doesn't have to replace sample one, however you'll want to make sure you choose the same deck featuring this gem when you start the match. The character I did this with was Yoshimitsu.
Next, you should try going in to versus. It's preferred you have two controllers for this, however it's not necessary. Go in to versus mode and choose a character BESIDES Yoshimitsu to go first and then select him. The other team doesn't particularly matter, just make sure you set your own handicap to 25% and start the round.
Once the round starts, move towards the opponent and double tap down along with the medium punch and kick (Triangle and Circle using standard controls). This will activate pandora, thus giving you a full super meter and switching you to Yoshimitsu. Now, you simply attack the first controller with your second and it will automatically block eating up 2 bars of super. Then use Yoshimitsu's Half-circle back + Any punch move (He twirls his sword, the input is right, down right, down, down left, left if you're starting on the right) attack to build up your meter back up to 2. Quickly attack again with the second controller, then let the round time out. This will net you one Pandora activation and two assist gems used. Repeat the process 50 times and you'll have your trophy!
If you're doing this against the AI, you can use a fairly similar strategy only it will be much trickier given the fact you can't force attacks. The strategy should remain the same however. You only have roughly 10 seconds when Pandora is activated, so make sure they count! You can check your progress in the player data along with anything else.