What range of levels is a good one to fight against them?
All of my pokemon were around level 90 so thats a could place to be around
i was wondering about having them at lvl 88… but i wasn’t sure if that would be enough hahaha thanks!
My 'mons were at levels 83,84 and 85ish when I beat 'em, Granted I had a lot of Revives,Healing Items, Leech Seed and Curse. London was the worst,saved that one for last.
Swept them with their own medicine aka baton pass. Funniest part was against baton pass chick when her Smeargle tried to put asleep my substitute 
Also I think that E4 levels go up if you level up so you can’t actually over level them. So minimum would be 88
yes! i swept Reukra with lucario and charizard both at 91 hahaha and only with 1 Full restore hahaha i forgot shopping 
They level with you I fought them the first time in the early 90s and they had level 96 mons. I lost so I grinded a few of my mons to 100 and turns out the elite 4 now had level 100 too.