That's strange. I'm not that player level and I still easily take down the highest gyms I've seen, which are level 6-7. As long as you can beat the weakest monster in the gym you can attack over and over until the prestige is low enough that there is only one monster in the gym. At that point it is easy to defeat even a 2,500CP Dragonite with 6 crappy Gobats since you have 6 shots to wear him down.

The only limiting factor for taking down gyms for me is the number of potions I have. I usually only fight with half dead monsters so I can use revives instead, then I use potions for the one that I put in as a defender to conserve potions.