How do you level up quickly?

Battling trainers is your best bet. Look for trainers and battle everyone you find.

However make sure you have correct Pokemon for battling misty. Grass is your best bet. Not too difficalt to obtian at that point in the game either...

Okay well you see you just have to start by going through the game once you have beat the elite you should be at least lvl 60-70 and down so you can keep going through their or you can do it the extra easy way go to cinnabar island and talk to bill he should be on the left side of cinabar island below the gym he well give you a pass and a boat well come get you and take you to the islands were you can find many new things but do the missions untell you get a tri pass to the other islands that way you can get to seven island once you arrive on seven island go down to wards the bottom of your screen keep on the path tell you find a house go in side their should be a man with his chancey talk to him he teaches you a dance that heals all your Pokemon grab the luck punch off the table go back out side go up now you should have battled those 2 trainers up their the one with a chancey and the guy w/ a 2 egxutor and one tangla
Together they give you about 6000 exp for each battle you do (chancey 3000, exuator 1500,exgautor #2 1600 and tangla about 1000+ for exp)
It well lvl you fast tell about lvl 80 then you are going to have fun training harder then ever because it wont give you much exp but it's better then what the elite four give you.