My hardest was Shelly, when I first went into that battle I had high hopes...dear god I was dumb. That Rain Dance set up is a pain, Anorith with Rock Slide wipes out any hope for Flying types, Wormadam's Steel/Bug typing, and Heather the Yamega...the soul reason I both hate and love Yamega now. I love Shelly to bits, but I despise her as an opponent.
As for my easiest...I would have to say either Florinia or Adrienn. For Florinia I had Trubbish, Tranquil, and Pancham at the time so that battle went by very quickly, Adrienn I had a chance to prepare myself and with the help of Garbage the Garbador and my newest edition Metallica the Shiny Bronzong with Heavy Slam and the Heavy Metal ability it went by with no problems.
I'm 50/50 for the next three. So here's to the future and the pain of battling.