Normally for me it was Norman except for one time. I sent out Zizagoon so I could heal one of my other pokemon but I forgot I didn't have any potions so I just decided to use rock smash and maybe take his defense down a bit. The brilliant thing was that Normans last Slaking kept using Focus punch and every time I used rock smash it flinched because of it.
So I would hit it with rock smash and it would flinch, Defense gets lowered, Loafing around, Rock smash, Defense gets lowered, Focus punch, Rock smash, Flinch, Defense gets lowered etc. etc. etc.
Then eventually I defeated Slaking with this level 12 Zigzagoon! I'm not sure if it was a glitch or something but I thought it was weird that it only used Focus Punch.
Also another Gym leader I found hard was Tate and Liza, the time I went against them doing a solo run with Blaziken was a nightmare!