The Cerberus enemies were heavily featured in the campaign, even more so than the Reapers, so they should be the most familiar of the enemies to you already. That said, they are also the most crafty in a number of ways. Just about all of their units will take cover, and their Phantom and Nemesis units will attempt to flank you for quick kills. This makes them one of the most dangerous factions, despite being one of the most familiar as well.
Their regular troops consist of simple Assault Troopers. These are the basic Cerberus enemies who you have no doubt killed hundreds of times already. They take cover efficiently, and they typically pack Assault Rifles. No big surprises here. The Centurions, however, are much stronger than the Assault Troopers. They have shields and Mattock Assault Rifles, which are more powerful and accurate than the Hornets that the Assault Troopers pack. Go for headshots or close-up Shotgun blasts when facing Centurions.
Guardians are another tricky enemy type. They will carry shields in front of their bodies to avoid direct gunfire. In the campaign, you had to kill these enemies by Pulling away their shields or setting explosive traps for them. Those are still viable strategies here, but it’s also much more feasible to flank around them thanks to the other three players who will have your back. The Combat Engineer is another enemy that will benefit from having multiple players present. These enemies would often be able to set up turrets in the campaign before you could reach them. Now, if you and a teammate or two target the Combat Engineer together, you can easily take him out before he sets up turrets. If you don’t see him right away, though, he can really become a nuisance. His turrets can chew right through the armor of weaker species like Quarians and Salarians, so leave the heavies to deal with this while you support from the back if you are playing one of these races.
Cerberus has three special units that will give you some trouble online as well. First up is the cloaked Phantom. Just like in the single player, the Phantom is capable of getting an instant kill on you with its sword. Unlike in the single player, however, you will have teammates to revive you. This makes taking on the Phantoms a less daunting proposition, so Vanguards and Sentinels can feel free to charge in and take them on knowing that their team has their back. Nemesis snipers will also pop up once in a while. These enemies are best left to the Vanguards on your team, as they are basically defenseless up close. If you lack a close range specialist, you can try distracting them with one player, preferable a sniper of some sort, while another player flanks around them.
The Atlas mech is the third special Cerberus unit. This one will only start showing up towards the end of each match, because they are very powerful. In the single player game, your strategy for taking them on may have been to kill the pilot and hijack the mech. It was a great strategy then, but it sadly won’t work in multiplayer. When facing one of these enemies, your team absolutely has to split up and tackle the enemy from all sides. Have one player hit it from the front, others from the sides, and another from behind. The Atlas won’t know which way to turn, and soon it’ll be toast.