The flags did me in a real treat, although I tried getting them as I went round.
I would prefer to have achievements like that the treasure hoarder one done before I finish the main story I feel I don't always want to go back and do things afterwards. Same for all the side missions, I did all of them before I continued.
Send me a message if you can help with the following;
Gears of War 3
Streets of Rage 2
Black Ops 1 & 2
Currently Working on the Gears of War 3 Campaign