Disgaea 3: Absence of Justice

In the item world there is a chance that at the end of the enemy phase a new group of enemies (pirates) will appear. They are all high level, so don't even look for them until your characters are maxed out. Usually if they do not appear after the first enemy turn it's not worth waiting; move on to the next floor. Also, you'll want to be looking for them on high floors (60+) as they have a better chance of appearing. In general, you'll fight harder versions of pirates in more powerful items, so you should stay in weak ones. At the end of the day whether or not you find them comes down to luck. You need to be in the Land of Carnage on floors above 90 to find Prinny, 10 Gents, and Baal pirates (though any of the pirates could appear under these conditions).

One strategy if you're not worried about illegal tickets is to get a weak legendary item to level 90. Exit the item world and save. Keep running the last 10 floors by reloading your game until you get all the pirate trophies. The pirates will be relatively weak ("relatively" of course meaning stats in the hundred thousands).