This is taken from my walkthrough for this site.
-Break Into Miraz's Chambers: In order to proceed, there will be a few puzzles that need to be completed. The first one is a ball maze on the ground. Use the directional plates to get get both the green and the red balls through the maze and to their holes. Go down the steps and go right towards the big door. A cutscene will occur. Afterwards, you will see a chest in plain view next to the door for 4 keys (Chest 10 of 16). There is an arrow icon to the right (in the air to the right of the door). Shoot it, and grab the crank that falls. Put that crank into the place it needs to be, which is in the next room above the cage that's in the middle of the room. Turn the crank, which lifts the cage up. Shoot the ball off the stand (there is an arrow target). Pick the ball up, and take it to the room towards the bottom of the screen (there's an archway to get to the next room). Put the ball in it's appropriate spot on the 3-handed statue. Head back to the previous room, get each of the other balls, and bring them and put them where they need to go. This shows a cutscene if done correrectly. In the room you're in now, a door has opened to the right, so go through it. Head up this long hallway, defeating the guard. After a few steps down this hallway, you will see a huge picture hanging on the wall. Destroy it, and behind it, there is a chest for 3 keys (Chest 11 of 16). Head back the way you came in (towards the screen), past where you entered the area, and come to a door which requires Peter's grappling hook to open. Kill the lone guard, grapple the door, and then enter it.
Rescue Cornelius From the Dungeons: After a cutscene, head down the staircase. Turn around, and follow the back wall until you reach a chest for 3 keys (Chest 12 of 16). Right beside the chest are some crate/barrels with an iron mixed it. Break the stuff, get tthe iron, and place it on the glowing area right above you. Get some keys. Now go towards the door with the arrow icon next to it and shoot. A cutscene occurs, showing a big, bad guard. Stand in front of the open cage, and when he charges you, move out of the way. This will lock him into the cage, and open the door right in front of you. Do to these two guards what you did with the last one, and get them trapped in the smaller traps. After they are trapped, head to the right and see a gate with two glowing chains on either side. Walk up and pull one, while your partner pulls the other. Go inside, and get the chest for 7 keys (Chest 13 of 16). Now, move towards the screen and find the gate with the arrow icon above it. Shoot it, go inside, and pull the lever. After the cutscene, go out and grab the crank that fell. Take the crank left, attach it, and turn it. This rises a gate. Move up and to the left and enter this room. Open this chest for 2 keys (Chest 14 of 16). Grab the lever right beside this chest, and head to the right hand side of the room to attach it to where it goes. Pull it to move the guard's cage closer to the door. Do this with with the lever on the left side too. This opens the gate in front of you. Go through this gate. After the cutscene, all you have to do is get the two guards to run into the 4 standing wooden pillars. Each pillar requires two hits. After they do, you get a cutscene, and this achievement