For anyone who has completed all of the Assault Ops Challenges on another platform and synced their progress to Windows 8, you should still able to earn the Extra Credit and A for Effort achievements. Using the method listed below, I was able to obtain these seemingly partially discontinued/unobtainable achievements. Keep in mind this method will erase your save progress for the Windows 8 version of the game and the progress might sync over to other versions if they are played after deleting your save.
-First, delete savedata.bin from the following directory: C:\Users\[Username, check note below.]\AppData\Local\Packages\Microsoft.HaloSpartanAssault_8wekyb3d8bbwe\LocalState and inside the 2533274810843141 folder. The 2533274810843141 folder may be a different series of numbers, but will have the savedata.bin file.
Note: Make sure to replace [Username] with the username connected to the PC user account where the save data was created.
-Go offline and start Halo: Spartan Assault. Once loaded, your progress should be currently reset and you should still own the achievements previously unlocked.
-Complete at least one mission while offline to create a new local save file.
-Now go to main menu and go online while in game.
-Complete another mission to be safe. Then, go check your stats to make sure everything stays reset.
-If done correctly, your progress should be reset and you will be able to earn Extra Credit and A for Effort by completing the necessary Assault Ops. (A similar method may work on Windows Phone, but I have never tested it.)
Thanks to MasterLerwing for additional advice for getting this method to work.