We’re working hard on eliminating the bug that is preventing people from getting the “Realistic Difficulty” achievement for Splinter Cell Conviction (Complete Single Player Story on Realistic Difficulty).
While we can’t discuss timing right now we can confirm that this issue will be fixed in the next title update for SCC.
The fix will automatically detect if you’ve already finished the game on Realistic difficulty and award you with the missing achievement. This means you will not need to play through the single-player campaign a second time after the update comes out, it’ll all be handled automatically.
We apologise for any inconvenience this may have caused and thank you for your patience while we work to resolve this issue.