Lets just think about what chuunin level means. From what we've seen of chuunins in an actual fight (Iruka/Mizuki) they are pathetically weak. Can you imagine if Iruka was taking the chuunin exam the same time Naruto and co. did. If by some miracle his team made it out of the forest of death then he would have been slaughtered by almost anyone he faced in the prelims. Forget the fact that he would have had no chance against any of the finalists. Therefore, strength is not a significant requirement for becoming a chuunin.

I don't think anyone would deny the fact that physically Shikamaru is weak. He did prove his intelligence and analysis skills however and this is why he is now a chuunin.

Unfortunatly for Shikamaru, unless he gets alot stronger, he will never become a jounin since you definitly need both intelligence and power to achieve that rank.