Bring a Electrode to lvl 100 and hold the move, Explosion. Or catch him higher then lvl 100 in Cinnabaar Island. he has also the move Explosion already. Remember: The missingno trick is required. Next, make sure you have enough Revives or Max Revives, copy one of this item will be helpfull. Next, go to the Pokemon Leaugue and start a fight against the first Trainer at the Elite Four.

Step 1: The first thing you must do: is to choose a Pokemon you want to train.
Step 2: Send your Pokemon out for battle.
Step 3: Switch to Electrode.
Step 4: Command him to do the move Explosion. Effect: the enemy Pokemon will sure die and Electrode also, in any chase, your first Pokemon will get all Experience from the Enemy Pokemon, without shring it with Electrode. If not, Electrode dies only, and your enemy will survive but... he has lost a major HP so it will be easy to defeat by your first Pokemon.
Step 5: Use a Revive to heal Electrode.

You can repeat this trick, as much if you want.