This trophy is only available after Swaine joins your party (which occurs during your first visit to Castaway Cove), but it's pretty easy to get once you have him.
Once you have him, then all you have to do is take control of him during battle (using L1 to select him), navigate to his Trickshot abilities, select Mugshot and choose your target. It's a low 1 MP ability that's available as soon as you have Swaine, so no worries about it eating up a bunch of precious MP.
If the Mugshot is successful, you get a notification immediately telling you what item you got... and you're 1/50th of the way to the trophy.
In my experience so far, I've found that:
a) You can only steal one item per enemy (please correct me if I'm wrong)
b) Your best chance to steal is after the enemy has been wounded. I never had any luck stealing from a full health enemy
c) Be mindful of your team members' tactics and/or how quickly they're beating down your target. Because the Mugshot animation takes a few seconds to kick off, by the time Swaine fires, the enemy may have just been defeated by your team and you get no item.