The key is being patient. The mission Numbers is relatively easy, just use cover a lot.
On Project Nova, you have to be patient and let your teammates move up sometimes and try to kill as many enemies as they can. The last part is kinda hard, so just use cover in that part and sprint for the finish at one point.
On Victor Charlie, same thing- use cover a lot. Use the Grim Reaper for the boat, then save it for later harder parts, such as the MG emplacement soon after, which destroying with the Grim Reaper will get you an achievement. In the tunnels, be constantly aiming a lot, and pause before turning into a new corridor.