I see a lot of people post these things, and I figured I would as well! My team switches up like CRAZY, though, so I'm just going to show what I used for Serra's gym battle. I beat her on my first attempt, and it took about five or six turns
Magmar/KxR xN @ Bright Powder
Lv. 48
Quiet Nature
Lava Plume
Faint Attack
Strength
Confuse Ray
Scrafty/Amethyst @ Red Card
Lv. 43
Serious Nature
Brick Break
Swagger
Headbutt
Crunch
Gyarados/Cr4zE @ Quick Claw
Lv. 44
Adamant Nature
Dragon Dance
Aqua Tail
Bite
Ice Fang
Gardevoir/Noxie @ BrightPowder
Lv. 46
Hardy Nature
Calm Mind
Double Team
Psychic
Magical Leaf
Magnezone/Jim Bob @ Focus Band
Lv. 45
Lonely Nature
Spark
Flash Cannon
Metal Sound
Electro Ball
Ninetails/Kiki @ Leftovers
Lv. 43
Rash Nature
Flamethrower
Confuse Ray
Will-O-Wisp
Hex
Essentially, my strategy was switch Scrafty out for Drought Ninetails first turn, then Lava Plume the snot out of Serra's and my own Pokemon alike until she went down. I only lost Ninetails and Gyarados during the battle, and I didn't have to use any Potions or anything. It went pretty well!