I know this thread is old, and I don't know if you're still monitoring this, or for anyone who wants to know, since thunderstormer failed to answer your question...
yes, you must start a new campaign on hard, but you can use the cheat codes for invincibility for you and your team and max weapons and still get the achievements. You must input the codes at the beginning of every mission, though.
It can be a little awkward entering the codes, but here they are (from the Cheatcodes that still Unlock Achievements
Ghost Recon - Advanced Warfighter:
(Note: Pause, hold down the buttons in this order:
. Keep held and enter the cheats.)
Unlock all Levels:
(Note: Resets when weapon is changed.) - what this basically means is that if you change weapons at any time, all cheats are reset, so pick your weapon before you enter in your codes.