Here is the full list of all 14 William and Sly: Classic Collection achievements.
(W&S 1) Turn off the rain
(W&S 1) Slay 5 Darklings
(W&S 1) Find all the mushrooms
(W&S 1) Unlock all boxes and switches
(W&S 1) Complete the game
(W&S 1) Find the wings
(W&S 1) Earn more than 35,000 points
(W&S 2) Find 50 mushrooms
(W&S 2) Find all the mushrooms
(W&S 2) Learn to fly
(W&S 2) Flame away
(W&S 2) Find 5 journal pages
(W&S 2) Complete the game
(W&S 2) Open the runes