When you are on the attacking side, the most convenient tool is the spawning grenade. Throw it near the area where you have to go, so your team can be lazy, and at the same time you can easily get a Ribbon for having 5 people spawn on your grenade. Swarming a room is also very useful.
When you are on the defending side, the Engineer badge is the most useful. Place turrets overlooking the target, and repair ammocrates and turrets nearby for your team's use. Alternatively, C4 Explosives are useful for defending entrances.