this might sound a bit risky and rewarding, but one of my favourites to use against the E4's Bruno in particular, also fairly effective against agatha; none other than, Butterfree!
now, the real surprise is the moveset to use.
i've found running butterfree with Sleep Powder, Dreameater, Shadow Ball and Giga Drain, to be alarmingly effective. even more remarkable that such an early-game pokemon can turn into quite a problem situationally.
i hope that helps offer up some interesting ideas :)
It is a wizard's prerogative, to be perpetually grumpy.