taking down high level gyms

Yes first I use up all my evos that week that have too high CP to be prestigers. Then I roll out w/ my attack squad and take over. This attack squad is a work in progress w/ power ups pending on walking buddy progress such as Tyranitar & recent addition of 2 Machamps due to Blisseys..

Say I'm attacking a Snorlax with my SW/O Dnite and I know that the next opponent will be a Dnite. At the end of the battle, I'll save my energy by not using the charge attack. Then when the next Dnite comes in, I'll release two Outrage in a row. KO in 7s.
The point is saving charge attacks which can be SE to next opponent.

Battle in group for sure. When we attack a gym, with 3 players, we can take down any defender in max 20". It's a lot faster and save more potions/revives than when you battle alone.
Last night we joined a group of five trainers, and took down two gyms, both were Blissies gyms. No Blissey lasts more than 10" or could release her charge move.

What s0measnguy and bioweapon wrote, plus:

Often I would have an abundance of potions but not revives so what I try to do then is swap out mons before they faint keeping them all alive.

And of course, look out for that type advantages. Always battle strategically.

I'm same level as you. To make it quicker and save my time I always use pokemon that are SE against the defenders.
To save revives, I don't let my guys faint. I find more efficient to spend big potions on them than revive them aun use small potions.
I noticed that prestiging is more potion/revive consuming than battling the gym.

sometimes its better to battle just the first mons when they are much weaker to the rest.

gyarados 1 + gyarados 2 + gyarados 3 + gyarados 4, then dragonite, jolteon or rhydon depending on matchup...although the AI wants me to use my 2200 venusaur against everything.

My three dragonites take out any level 10 gyms I want. If there is more than one Blissey I'll bring a fourth one :)