To get 100% completion you need to:
Complete all tombs
Complete all missions
Complete all challenges
Collect all collectables
Complete the story
Playlist with all collectables, how to complete tombs/challenges (not mine Credit to 360GameTV) :https://www.youtube.com/playlist?list=PLuGZAFj5iqHcQMAD725ZJ...
Link to his thread for little more info on what gear is needed for certain collectables:http://www.xboxachievements.com/forum/showthread.php?t=63247...
If you try to get 100% in your 1st playtrough then after completeing the story after the credits it will say 98% then in main menu press continue watch the cutscene and the achievement will unlock when you gain control of Lara