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make sure you buy between 10-20 max repels and you need Pokemon that know surf, rock smash, strenght, rock climb and waterfall. then work your way around don't worry about the trainers there just bring your starter Pokemon, dialga and your strongest Pokemon that know the HMs (cause to be honest there all week i did it with just a level 50 dialga and a level 53 empoleon). good luck the max repels will last you till you get out cause its the shortest one yet.

get a whole bunch of max repels, get your strongest pokemon and than avoid trainers as much as you can. oh and make sure you have a pokemon that knows a lot of HMs cause you will need them(Rock climb,Strength,rock smash)bibreal is always good.

buy loads of max repels, bring pokemon that you have taught hms to, and avoid other trainers as much as possible. make sure your pokémon are high levels.

go to serebii.net or gamefaqs.com