Operation 40 and Vorkuta are not that hard to do, just take it slow, use cover and let the AI players kill some of the enemies, they will constantly rush forward most of the time. Don't save and quit in middle of a mission, cause the achievement might not pop because of that. This happened to me.
In the end of Executive Order there will be a long part with no checkpoints, you need to get to a room on the right side of the corridor and the enemies will respawn continuously, save some smoke grenades for this part.
The enemies can't respawn when they are alive inside the smoke cloud, also they mostly won't shoot you. I recommend saving at least 2 smoke grenades for this corridor. Throw the other one after you have reached the room on the right, then you will see a small room on the opposite of this room (far left of the long corridor). Lob the other grenade there and rush to this room. You might hit a checkpoint and after that, it is easy to finish the mission. Note: in this room there is usually a couple enemies there, so be ready.