How to Mod-
*Dark Sector - Xbox 360*
1.) Open your save file "UserSaveContent" in HxD (or any hex editor).
2.) Do a Text-String (non-unicode) search for "achievementprogress" (without quotes).
3.) Under "achievementprogress" you will see several entries that look like...
{. name=HEADSHOT. value=29. }
...This is your achievement progress in the game. You will only see the entries and values for the achievements that are in progress. (ie, once you do something towards an achievement it will show up).
4.) Now, go through and edit the different values to how many you need (enter a value that is one less than what the achievement calls for, so if it's for 30 kills - enter 29, so you get one more kill and the achievement pops!).
5.) When you make changes to the entries you'll have to keep up with how many bytes you've added or subtracted. If you've added or subtracted any bytes you'll have to change the size-check block (I haven't found where this is located yet). Or, the easy way is to just add or subtract the bytes/change the values in different entries.
*For Example if it looked like...
name=HEADSHOT value=9, name=SHOOT_PROJECTILE value=10
*and if you needed 30 headshots, change it like so...
name=HEADSHOT value=29, name=SHOOT_PROJECTILE value=1
So...we added a byte to the headshot value, and subtracted a byte from another value.
6.) After you've edited the save the way you want, Save the file.
7.) Then Rehash the file.
8.) Transfer to MU/HDD.
9.) Start the game, and load your save or continue.
NOTE: This tutorial is assuming you only have one save-game on your save file. If you have more than one save-game, you'll have to search through the file more because there will be multiple sections to edit.
Credit Goes to JizzaBeez