These missions are not too difficult, some of them are even easily doable with just a few players where as some may require several for it to go smoothly (such as Swamp Airfield) so the more players you have the easier it will be. An important thing to remember is that if you die in the mission, you can still earn the achievement if the other players are still able to complete the mission, just make sure to stay in the game and spectate until it's finished. These missions take from 30-90 minutes depending on which map and how many players you have in your lobby. I will provide a few vital tips to what I think is worth keeping in mind in order to pass these missions without trouble.
- Don't rush..You have the Drone. Personally this is the most important resource you have in the game (maybe the only proper support) which brings me to the point that you must not underestimate the enemies as they will come out of anywhere in huge numbers and will instantly kill you if you're not aware of them, therefore always using the drone to identity enemies on the minimap will save your life countless times and make the whole mission much easier. Always move the drone ahead before you move so you're prepared and don't rush unless you have other players supporting you.
- Keep your friends close. Like mentioned before, the more players you have in the game the better. If you're are only playing with another person you may get outnumbered at times and the missions will become more difficulty than they have to be. Try to not go too far apart when playing in case someone goes down and no one is close enough to revive them as this has been the case many times through personal experience, these situations aren't necessary if everyone moves up each part of all the maps together at a steady pace.
- The right Loadout. Make sure you and your whole team spawn with the right equipment/guns. Having 4 Heavy Gunners on a sniper missions will not get you anywhere. Suppose you're in a team of 6 people, it would be most ideal to have 2 Snipers, 2 Attackers and 2 Marksmen so you're able to deal with any situation and enemy including the deadly tanks/APC's & choppers that you'll encounter several times!
- Remember Secondary objectives. These can include various tasks such as clearing out a camp full of enemies or taking out a Chopper. They are all required to be beaten on all the maps to unlock the achievement, so pay attention to the missions brief, in game map and objectives as you may often miss or forgot a certain objective. You can't progress through a mission before you completed the main objective anyway so don't forgot the others.
You can find partners for the missions in the forums.