I did this in Episode III, Chapter 4 (Defense of Kashyyyk). The third area has two droid commanders endlessly calling droids 12 at a time, which gives you a nice supply to work with. I recommend using one of the ghosts and a bounty hunter that's not too many clicks away in free play mode -- I used Ben Kenobi (Ghost) and Greedo.
As the ghost, the droids will mob you but not attack. Let all of them get around you, then quickly switch through the characters to the bounty hunter (for me it was 2 left buttons away) and immediately throw a thermal detonator with the B button. As soon as it hits a droid, it will detonate and take out most of the mass of droids -- and probably you as well. That should be enough, but if not, keep the droid commanders alive and you'll have unlimited droids to try with.