Answer 1

I found a pretty simple way, using only rentals.

1.Haunter
2.Wobuffet
3.pokemon super effective against gym leader or counters their strategy (ex. using safeguard against Erika)

Use wobuffet to counter or mirror coat the foe, taking out at least one, maybe 2 if you can get in destiny bond. If one particular pokemon gives you trouble, use haunter and destiny bond another, then just sweep their 3rd pokemon with a super effective hit. This worked really well for me, beating almost every gym leader on the 1st try.

Comments

That's why I like Wobbuffet most!

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Another way is the way I used. If you are able to, get your team from Gold/Silver/Crystal (making sure they are all equal levels.) Not only do your Pokemon have movesets that work great, better items, and EV trained the way you want, you can also get in some Pokemon that aren't available as rentals until later in the game: Lugia, Ho-oh, and Mewtwo.