I'm on my second playthrough so I did have some memories about their strengths. I however don't use any potions or other healing stuff during battles to raise the challenge.
Julia, Flobot, Corey, Shade - Got them all on my first try after careful preparation. It was really close with all of them though, I only had one pokemon left each time. Nearly got a heartattack when Corey's Crobat nearly OHKO'd my last pokemon. All of these battles were well-balanced and decently hard. Loved it.
Shelly - She never used Rain Dance so it was dissapointing to see her struggle on the burning field. Easy win.
Cal - Is he some kind of joke? I even trained a Camerupt to use against this guy but in the end I didn't even use it. He was totally weak.
Kiki - Flying types don't work because of accuraccy plus psychic doesn't really work against her. Medicham, Galade and Lucario don't have the psychic weakness and Toxicroak just Sucker Punched my Jynx that I've trained especially to use against Kiki. So much for that. She litterly mopped the floor with me like 3 times without even use her Medicham.
What pokemon and strategy would you recommend against Kiki? I'm really clueless what to do with her. Ame's advice to use psychic types has proven to be totally useless. I mean I did beat her eventually when I knew all her tricks but unlike the previous leaders this victory was not deserved at all. I want a rematch!