In my latest E4 beating in LG, I beat them with 2 pokemon:
Lv.58 Charizard
Lv.58 Gyarados
I think those are the levels that I were at when I started battling them (first wave) but in FR I can beat them w/ a lv.100 Groudon,(second wave) but I rarely do that anyway.
I always carry 6 pokemon the first time i come to the pokemon league, but i have beaten it in emerald with only Kyogre Lv. 90-100. I beat it many times to get kyogre from 90 to 100.
On my Emerald, I used my Lv.100 Rayquaza to raise my Lv.22 Trapinch. Trapinch (now Flygon) was in the Hall of Fame but Trapinch didn't use a single attack at all. Rayquaza did all the work.