It's eternitys ago, but I can remember I once beat the Johto Elite 4 with a mid-high 30s team. Don't remember much details, only that I had a Kadabra and/or Jynx in it that surprisingly easily beat Lance.
Well not using to battle but rather to gain exp with was my Ponyta.. Trying to evolve it into Rapidash is such a pain..
I think it was level 30 or something.
The lowest level I battled the E4 with was a Luxray level 40 something, and now it's one of my highest level Pokemanz =3
I don't know if I've ever had any in the 30s, but I don't think so. But almost all my Elite Four teams are in the mid-40s just to make it more challenging, so I'm not sure which one is my actual lowest.