Your basic objective during all Mass Effect 3 online matches will be simple survival, but several waves will add further objectives on top of that. These often revolve around hacking terminals or uploading viruses, but can sometimes be a little more specific and challenging. These special objectives will appear during the third, sixth, tenth, and eleventh waves of each match, so be prepared with stores of ammo and equipment.
The first of these mission types is Assassination. This will typically appear on the third wave. In the Assassination variant, you are assigned several targets who you must kill before the enemy forces will cease attacking. The targets of the Assassination are completely randomized, but since this typically appears on only the third wave, you won’t have to worry about having to destroy an Atlas mech or a Banshee or anything crazy like that. Oftentimes, the targets will be Nemesis snipers or Phantoms. If an enemy is targeted for assassination, they will benefit from increased strength and shields compared to most units of their class. It is still a relatively simple process of taking one down by yourself, as long as you can get them cornered.
The next mission type will typically consist of enabling or disabling several consoles set up at various points around the level. You will have to proceed from one waypoint to the next, planting or removing the devices as you go. When you go up to the objective, hit the action button to begin the process. The process will take a short amount of time, and you will be defenseless while it happens. You will also not be able to enable or disable more than one device at a time. For these reasons, you should try to move your squad around as a cohesive whole while this process is occurring. They can cover you while you enable the devices.
The next mission type is Hacking. This plays out pretty similarly to Enable Devices in that you must proceed from waypoint to waypoint accessing terminals. The hack will take a while to complete, longer than the Enable Devices missions did. The more players inside of the little blue circle around the terminal, the less time the terminal will take to hack, though, so again try to travel as a unit during these missions. Just be sure to keep a defensible position while maintaining the hack. If you have a Quarian Engineer, setting up a Turret is a great way to survive this wave. Placing Proximity Mines with the Turians can also be really helpful.
The final mission type occurs on the eleventh wave and is called Extraction. This is a simple survival objective. The dropship will arrive in two minutes to carry your off to safety. You merely need to have your whole team in the LZ by the time the timer is up. If your entire team doesn’t make it out, you will all suffer heavy penalties. The strategies for surviving the Extraction mission vary depending on the map, so those will be covered in greater detail in further, map-specific guides.