In all honesty, Aya was the only leader that legitimately gave me trouble. Maybe it was just my team, but I actually swept through most of the other leaders on the first or second try.
Julia: She was very kind in powering up my lightning rod Electrike for me.
Florina: Kriketune (probably misspelled) set up and destroyed everything.
Corey: I don't remember what I did here exactly, but it swept through quickly and I moved on.
Kiki: Gothita and Noibat won this one for me, along side my Greninja who baited her into using moves like HJK, then used lick to turn into a ghost.
Aya: I ended up completely revamping my team. I trained a Flygon, Sandslash, Klink, Magneton, and Shedinja just to make sure that i could beat her.
Serra: I fought her in Episode 12, so she wasn't as difficult anymore, so my Pyroar and Magneton plowed on through.
Noel: I had a Scrafty which decimated. I also had Magneton and Manectric to kill the birds and Clefable.
Radomus: I trained up some new Pokemon here, but that was simply because I knew I'd be leaving the area for a while. I grabbed Aggron, Gardevoir, Archeops, and Mawile here. When I went into the gym, it was a cake walk.
Luna: Gardevoir, Scrafty, and Pyroar swooped in from above and 6-0ed her.
Samson: At this point I did some training and rotating out because 5/6 of my team were weak to fighting types. I trained Archeops and grabbed Hawlucha. These two combined with Gardevoir won it easily.
Charlotte: After fights that are meant to force you to think, I was kinda disappointed in this one. Rain Dance+Archeops and Flygon spamming Rock Slide kills everything.