Cerberus has a couple of very powerful boss enemies that each can instant kill you if you get too close to them. The phantom is very annoying when it gets close to you and has caused many teammates to die. Atlases are difficult to deal with because their approach is slow but relentless. They don’t usually kill you by themselves but cause you to stagger which helps other enemies to kill you whilst you are defenseless.
They are only a problem when they are close to the main body of your squad. Otherwise they sometimes shoot rockets at you which you can easily avoid by going into cover or barrel rolling just before they hit you. The rockets do area damage which can be a pain of your squad is bunched up close together.
Teammates often take care of them with missile launchers, but Atlases aren’t actually that hard to kill. Overload, Energy Drain and other biotic powers that are good against shields can easily take care of the Atlas’s shield and even set off biotic explosions. I usually like to focus on other enemies during cool down of overload and use it on the Atlas after the cool down. When they are down to their armor use warp together with other biotic powers to cause biotic explosions which will kill the Atlas quickly. If you are using guns be sure to use machine pistols for the shield. Slower shooting weapons like the sniper rifles and heavy pistols are very effective against the armor. Be sure to shoot the glass for some extra damage.
The Atlas has an instant kill move. As a vanguard it is usually safe to charge if it is doing its ground stomp or if it has its back to you. During those times it can’t instant kill you. It is also a safe bet to charge it when it has 2 or 3 bars of armor left because you will kill it faster than the Atlas can grab and kill you.
Line of sight is also very powerful when versing an Atlas. If you can’t see the Atlas’s guns it can’t shoot you. Keep low cover between you and the Atlas and you can shoot it whilst it can’t shoot you. Vertical cover can also be useful because as long as the Atlas is in your sights you can shoot it even through walls:
Phantoms are annoying because they cloak and often get close to you without your knowledge. The phantom’s sword attack can stagger you and they also have an instant kill move that sometimes is triggered when you are close. If they are crouched no biotic abilities except charge will work against them.
The vanguard is very good at killing the phantom. The charge and nova stagger it which will stop the phantom from instant killing you. If you are an Asari Adept stasis is very powerful because it keeps it locked in place for 10 seconds or so and, if you use another biotic power, it will cause a biotic explosion. Be sure to kill phantoms before they get too close to you. They are weak at a distance.