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I defeated Ghetsis with my level 48 Serperior which was the strongest pokémon of my party and decided to switch my Zoroark with Reshiram when I caught it. My Serperior was also the strongest at level 67 when I defeated Alder. I think 5 days is good It took me 22 days to complete the story and defeat Ghetsis and 20 days later I defeated Alder. Did you switch any of your party with Zekrom?
I also have exams soon. In Black version I have defeated Alder and I'm now trying to complete the national Pokédex. In generation IV my current obtained number is 490. What part of White version are you at?