Julia and Serra
When i first played Reborn, i wasn't used to the idea of the first gym leader with 6 useful pokes. So, it took me a while to realize that i should be using more than... idk, my starter in the the first gym :I
Serra also gave me trouble, cuz i didn't use Sunny Day/pokes with Drought.
No problem with the other leaders, cuz i had Godrados and yeah...