This achievement requires you to achieve the maximum star rating in 20 missions or challenges. Just to avoid confusion this can be a mix of each eg. 15 Challenges and 5 missions with maxed stars.
Now I would recommend working on as much of your mission stars as you can, simply because the more missions you complete the more challenges you unlock to play. Personally i used this to my advantage as I could not max out a decent amount of the missions. But thankfully because i had plenty of challenges unlocked i used them to increase my numbers towards the achievement.
There are no videos that can help on YouTube (at the original time of writing) so you are on your own! I would suggest working on what ever you find easiest to start with. I find the challenges easier as they are pretty much dedicated to being efficient with just one weapon at a time and less soldiers, unless its a fast swap challenge.
Weapon challenges come down to how well you use the required weapon, normally very easy to max out. As for vehicle challenges they come down to how well you drive but in some of the challenges you may need to shoot extra targets for bonus points which will be required to max out the challenge. For the fast swap challenges you would be lucky to max the challenge out first time, this requires patience and practice so that you can instantly swap to the next guy for the fastest route once you can remember where the soldiers appear.
As for the missions, I find them a bit harder as they require more stars to be maxed out, which include Total Score, Accuracy, Weapon Efficiency, Soldiers who died, Time and a couple of other categories. All of the required categories need to be maxed out in 1 run of the mission, so if you miss out by so much as one star, you'll need to do the whole mission again from scratch.
The reason I had given up on this achievement is because for some reason my stars reset... three times!! Possibly because of mixing online play with campaign play, or because of when I sometimes when I clear my cache and recieved an update that may have wiped my stars clean!
Unfortunately I have no solid info on why they disappeared but as you can imagine I was not impressed! So my advice would be that if you are going for this achievement, try to do as much of the campaign as you can first before grinding the missions and challenges.
Good luck guys! This one will certainly not come easy!