Well, I have two main ways:
Fighting Chanseys and Elite Four Grinding
Let's take a look:
Fighting Chanseys
Pros
- you get a lot of EXP in the early levels
- doesn't require you to do much
Cons
- does get boring
- can end with you accidentally killing the original
- levels start to slow down after lvl 50ish
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The Elite Four
Pros
- interesting
- you get an extra fight at the end
- you can get EXP quickly
Cons
- if can be hard to defeat them with a weak team (lvl 50 or below)
OVERALL:
I'd go with both! Use Chansey to grind to level 60ish and then use the Elite Four to get to level 100!