the strength difference between a high-level genin and a chunnin isnt to high because obviously naruto and sasuke is in contention for a chunnin upgrade. Naruto and sasuke could probably beat most of the weaker chunnins and naruto could probably beat nearly all if he uses kyuubi. hell if anything bad happens summon gamabunta :lol:
B-rank mission is easier then the first mission they had and gaara was able to complete a B rank mission pretty easily.