Why try to duo dungeons for exp? You'd need to be tank and healer most likely particularly if doing it at the level that the exp would be worth it, and things would take far too long without dps.
Its not hard to get to 50 by mixing everything in. I took ninja 1-50 in 3-4 days of casual playing by mixing in my daily low roulette, first time bonus on guildhests, quests, leves, and doing fates in zones that werent popular. I ended up reaching 50 long before others who were lvling exclusively in the popular fate zones.