wrote a great guide that can be found here (scroll down to the Treasure Meister section [posts 8-10])http://www.xboxachievements.com/forum/showthread.php?t=10971...
If you prefer a video solution check out this playlist by Oddleif65
, it may be for PC but it still is the same:https://www.youtube.com/playlist?list=PL5A157D9C9A7D970EA few notes I gleaned from my playthrough:
-You're going to need a lot of keys. Try to grab them whenever possible. Personally, I only had to replay 3 missions to get more keys. Try to do these on the early levels before the game's mission structure becomes more linear. If you want to collect the chests after beating the game, the later levels will give you lots of keys.
-If you're wondering, keys carry throughout the entire game. Moving on to new levels and replaying old ones both will add to the key count.
-Specifically on the mission where you find the children's gifts in the second level, head to the left room before the top and right ones. The top and right ones contain bonus chests while the left does not. The left has a good number of keys though.
-I found the guide linked above very clear on all areas except one. For Aslan’s How (Help Lucy Find Aslan) #14, I misinterpreted the line "Before you jump over any barriers/rivers etc, look out for battlements ahead of you.". You may jump over barriers/rivers but when you see the battlements--in the area ahead of you--stop and look for the chest. You can also see it on your map if in doubt.
On your way to completing this you will unlock all the other collectable achievements. If you pull these off in order, you should unlock...
...at Cair Paravel
...at the Ruins of Cair Paravel
...at Miraz’s Castle Assault
...at the Battle of Beruna