The Reapers don’t have a main boss but a number more powerful enemies. Namely Banshees, Brutes and Ravagers. Banshees are very annoying because if they get close to you they will instant kill you unless your teammates stagger it when you are in the death animation (that is do a lot of damage to it). Brutes are annoying when there are a lot of enemies around and Ravagers can kill you very quickly with their attack and leave an acid bath behind for your pleasure.
The main problem that Banshees pose is that it pushes you around the map. If you were just facing a group of Banshees coming at you from the same direction you would be fine, it would just take a while depending on what you were doing. But they usually spawn in the later rounds and push you into other groups of enemies or corners. This is usually the way that you fail missions.
The other common way to fail a Reaper challenge is when a Banshee pushes up into the uploading/hack objective that require you to stay near them for around 1 or 2 minutes.
Banshees are annoying because when they get too close to you they instant kill you by sticking their arm through you. This is especially annoying if you are playing as a vanguard because you can’t charge a Banshee not get instant killed. Stay away from Banshees as a vanguard and take care of a different area of the map, for instance make sure that your team can back away from the Banshee whilst killing it without running into other enemies.
Banshees can also do either a single attack where they shoot a ball of light at you. This does dot damage (little damage over time) which prevents your shields from regenerating and can kill you if you are low on health. They can do the same attack as an area attack, but it is weaker than the individual attack. Once the Banshee lifts off the ground and gathers light around it like this:
Be sure to back away from it. If it throws the light ball at you you can barrel roll just before it hits you to avoid it or duck behind cover. It will change course to hit you, but as long as you get a wall between you and the ball you will be fine. Just before the Banshee throws the ball of light it swings its hand like this:
The best way to kill the Banshee is with biotic explosions. Guns don’t do a whole lot of damage to it. For the barrier use overload or energy drain do deplete it quickly. You can also use warp and another power for a biotic explosion but this isn’t as effective. If the Banshee is down to its armor be sure to use warp, carnage, incinerate or any other power that is effective against armor. When using warp be sure to laternate it with another biotic power to cause biotic explosions.
Brutes aren’t actually that difficult to kill. Using warp and another biotic power to cause a biotic explosion will down it quickly.
The Brute by itself will usually just take away your shield. I have never died to a brute, usually to other enemies around it that shoot me whilst I am staggering because the brute just hit me. When facing a Brute you want to make sure he is by himself. As a vanguard I usually charge the more basic enemies around the Brute and worry about the Brute once they are all dead. With other classes the brute is much more deadly because if it gets close to you it will stagger you. I usually focus on Brutes after Banshees because they are annoying when they are close to you.
Watch out for when Brutes fall over near you. Whenever that happens you will loose your shield, even after they are dead.
Ravagers don’t have a lot of armor, but they have a very powerful attack at the same time. If you get hit by it three times you are dead, at least on silver. On bronze you will be left with very little health and on gold you are almost dead after the second hit. Be sure to avoid line of sight with them. They aren’t very fast so they aren’t a high priority to kill. They only have armor so you can use any power that is good against armor, like warp and incinerate. Guns also kill it fairly effectively, especially sniper rifles and heavy pistols and some assault rifles.
After a Ravager has died it leaves a pool of acid behind. It will do dot damage to you, even after you step out of the pool. It is especially annoying if you are a vanguard so be sure to avoid stepping into it. S warmers also spawn after it has died and when you shoot the sacks on it when it is alive.