Rematching gym leaders is a great way. Also rematching the elite 4 is a very fast method, as their pokemon go all the way up into the 70s. If you're lucky enough to find an item called lucky punch, that increases the amount of EXP you get. But you don't need to be over leveled to defeat him Here's my advice for each pokemon based on your team:
consider using a move like thief/covet on pikachu, it has a light ball, so by stealing its item, you cut its offensive power in half. It also has very frail defenses, so a super effective ground move will take it down. If your Manoswine has it, that will make quick work of it.
for Charizard, Rock and electric moves work well on it. A water type if good too.
For Venusaur, your typhlosion or mewtwo can take it out.
An electric move will deal with lapras and blasoise.
Snorlax will take super effective moves, take advantage of its low speed.
You need a full team of 6!! you only have four pokemon, bring something else, like a water type or a fighting type to deal with his team.