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on voltorb, bring in sableye so you don't lose to explosion. mightyene should be able to beat electrike and combusken can beat magneton. Raise them all to about level 22 and you should be able to do it.

I usually level-grind my Combusken on the routes next to Mauville until it hits lvl 25 or so. That comboed with your other pokemon, a few super potions or sodas, and a little luck, should get you to sweep the leader. Roselias give out very good experience, and are obviously weak to Fire

Step 1: Get ground and Ice pokemon to around the level of his Step 2: Fight him Step 3: ??? Step 4: Profit