The above mentioned tips are good - safe often,use different starting locations,camp in safe spots.
Only go for the main objectives.
I would also add to the list that use the main character, as he has remarks like "I`m sitting ducks here" or "it`s time to go home" if you get spotted.
Listen to enemy whistle,officers tend to warn the others if they spot you.
I`ve managed to do 4 missions in under 10 minutes,mostly around 5,with minimal or no sniping at all,if you find it hard the long distance sniping.
Working on the others but will be hard to push them around 10 minutes or not use sniper rifle.
Save - Load is your best friend.
Mission 2 :
Mission 2 -
Stealth :
Mission 4 :
Mission 5 :
Mission 8 :