Hardest, in order:
Kiki - I fought her so many times that I knew exactly which pokemon I had to use against each of hers, which pokemon could survive a dark pulse from her Lucario, how many hits from my Ampharos it would take to kill her Scrafty, etc. It didn't help that my team was underleveled and the only pokemon who could damage her Medicham was Meowstic, who died in 2 hits. I think I fought her a grand total of 12 times before I beat her (I got close a few times, but somehow Ice Punch has a 50% chance of freezing my pokemon).
Shelly - Her Yanmega destroyed every one of my pokemon except Ampharos. I spent half my team killing that thing through burn damage every battle until I finally managed to beat her after she switched in her Yanmega on my Ampharos's discharge.
Noel - My team was about 5 levels underlevel when I fought him, so despite me having a Zoroark and Greninja, his Swellow outsped and 1hko'ed all of my pokemon. After some speed EV training I came back to find that his Swellow STILL outsped all of my pokemon. So I had to sacrifice my Ampharos paralyzing his Swellow, but luckily I still managed to win in the end.
Julia - This was mostly due to having a team full of glass cannons and nothing that resisted electric: Frogadier, Gothita, etc. Her Electrode basically outsped and killed everything until I took out half its health with a well-timed bide from my Pachirisu.
Easiest, in order:
Shade - Greninja and Zoroark spamming night slash made short work of his entire team.
Radomous - See Shade. I'm still curious why he didn't use trick room until the very end.
Corey - After I dispelled his field with a gust from my Beautifly, the battle was a cakewalk.
Serra - I lost two pokemon to her Cloyster, but afterwards most of her team died to Ninetails. It felt like a very easy battle, and I know there were more than a few times where RNG helped out.
Luna - My entire team was 5 levels down from hers, but Gardevoir basically swept everything with moonblast aside from Tyranitar and Bisharp after setting up with calm mind.
I have yet to fight Samson and Charlotte, but I expect them to be difficult.