The only Gym Leader I beat on the first try was Byron, but out of them all, I had the most trouble with Fantina and Wake. Granted, I made it harder on myself by only using the same amount of Pokemon as the Gym Leader. (I find it unfair that you can use more Pokemon than them.) I finally managed to take down Wake with help from Cut Rotom after about eight or nine battles.
But against Fantina...Oi. I went up against her the first time with Monferno, Togepi, and Gligar, but that didn't work. Next, I used Monferno, Togepi, and Zubat, but that still didn't work. Finally, after probably about twenty battles, I developed the perfect strategy. The only Pokemon I had trouble with was Mismagius, as Togepi could beat Duskull and Monferno could beat Haunter easily. Mismagius was my only problem. Finally, I found the answer to my problem in a Substitute/Foresight Hoothoot. I tore through Duskull and Haunter losing only my Togepi, managed to use Hoothoot to expose Mismagius to Fighting moves. Mismagius still almost took out Monferno, but a perfectly timed Mach Punch managed to win me the match.