Aaron: His bugs got smashed by my Staraptor. Drapion put up a modest fight, but was quickly defeated
Bertha: Only really had problems with Hippowdon. Apart from that, quite easy.
Flint: I hate his Infernape & Steelix.
Lucian: Apart from Mr Mime, quite easy
Cynthia: Quite appropriately the 'Champion'. She is (in my opinion) the hardest leader in all of E4 history. Her Milotic, Garchomp, Lucario and Roserade obliterated my team. Good thing I keep a large stock of items.
Overall, the D/P E4 was slightly harder than other game (harder than G/S/C and R/S/E, easier than R/B/Y and FR/LG). Cynthia was really the only difficult part of the experience, the other 3 were under strength and easy to deal with.