Haha! I never have that problem. I always have at least 1 Pokemon that are level 70 before I face the E4&Champion.
That reminds of how I played my first Pokemon videogame.
Ok, here it goes.
I play the game with my starter&never bother catching any other Pokemon.
Then I learn you need Pokemon for HM's.
I catch some but I don't even use them for battle. ^^;
The only reason I kept those Pokemon was for FLY,HM's I needed them so when my starter faints they back him up while I give my starter a revive&goes back into battle.
The E4&Champion are hard when you only use one Pokemon but I bought enough revives so I could win&beat them all.
I was ridiculously overleveled.
Even super effective attacks didn't hurt much.
But they sometimes killed my starter but I kept reviving him.
I never switched out so if one of my other Pokemon was still alive I use revives on my other Pokemon so they can back up my starter if he fainted again. ^_^
Probably I should do that again. XD