Hi, can you tell me the best way by bus from Penta-Hotel to Mutianyu

Well, the transportation is not that convenient.
From the hotel, you can take subway line 2 at Chongwenmen Station, get off at Dongzhimen Station.
Then take bus no.916 to North Huairou Road. After that, hiring a private car to get to Mutianyu Great Wall; the fare is about CNY30 per person.