Try to keep your deck as small as possible, less cards means less cards that need leveling. Every route has a requirement of cards, stick to that if you are underleveled as to not make the odds of getting your good cards lower. If you know you will lose the battle, don't go for the battle, either choose for the levelup, tribute or free card event if you can, if you really can't avoid a battle without buying berry/card packs or a glyph, go into the battle and forfeit it to save time. Rinse and repeat until you are at the correct strength.