Where and how should I level up to catch the Legendary Birds?

I haven't yet beat the Elite Four and would like to catch the legendary birds to help out with that. Where should I go to most efficiently level up Pokemon of the appropriate types? I intend to use a small number of type-appropriate sweepers for whatever level is chosen coupled with the majority of the team being low-level pokemon to be levelled up (either 1-5 or 2-4 depending on need). Is this a reasonable approach?

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