I have never beaten a Rocket leader and it's starting to impede my enjoyment of the game.

It takes their pokemon 2-4 hits to destroy even my perfect IVs. idk what I can do better.

They can be rough, yeah. How high is your highest CP pokemon?

Hmmm yeah, at that CP you should be able to withstand a few hits. I'd aim to take pokemon that can hit weaknesses or in a pinch have resistances

I looked up the best counters and used those, and still nothing.

Also they will auto shield the first two charge moves, so things that can fire off fast charge moves to break those down can help

My teams for the leaders tend to switch around and oftentimes I'll need to switch out pokemon before I find a team that can get the job done

If you want suggestions, I'd grab metagross as an option since steel has a lot of resistances and then go from there - I tend to not go with best counters but with what I have that's decent and gets the job done

A good water type might also be good?

yeah, my usual strat is to just open with something that can survive their first mon and has a rapidly charging secondary attack, and then once I've taken out their shields it's a little easier

Melmetal with Thundershock and Rock Slide has been invaluable

(Rock Slide's a good charged move to have on anything, really, it charges up super fast so you can burn the Rocket's shields quickly)