Mr.Vikou,
Which Leshan Bus Station did you mean, Leshan Xiaoba(肖坝) Bus Station or Leshan Central Bus Station (Leshan Bus Terminal)? The last bus from Leshan Xiaoba Bus Station is at 17:30.

From Xiaoba Bus Station to Leshan Buddha, you may take a taxi or take No.13 bus. The taxi will be faster. It costs around CNY16. I have no idea how much time you need to get to the top from the gate.

From Chengdu to Dazu, you need go to Hehuachi(荷花池) Bus Station to take long distance bus. It takes around 5.5 hours. The cost is around CNY 90. The last bus leaves around 16:00. Be ware that the environment around Hehuachi Bus Station is terrible. Take care of your belongings.

From Dazu Downtown, you may take mini-bus to Baodingshan. It takes around half an hour and costs around CNY3.