Question

Where is the best place to farm xp in Pearl before Elite 4?

Comments

Just battle the Elite 4 until you have enough experience to defeat them. Their Pokemon give a lot more experience than wild Pokemon do.

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Be it any game (usually till gen 4), fighting elite 4 without using any revives and lemonades gives you more experience and earns you some money in case you don't have enough of them.

Specifically in pearl, there may be some places you didn't visit yet, for example Rock climb near the exit of cave of Mt coronet, towards snowpoint  city, where you can battle 3-4 trainers and get TM ice Beam.

Answer 1

As sumwun mentioned, the fastest way to level your Pokemon is to grind them against the Elite 4. If you don't want to lose most of your money, your best bet is Victory Road. Be aware this is an inferior choice --- most Pokemon are in the low 40s, compared to Aaron's Level 53s and above.

Money is plentiful and you should have no trouble regaining it after losing to the Elite 4; refer to this question for details.