The Swarm is the major antagonist of the game, and is a race of highly parasitic aliens that appear to consume all living organisms for sustenance, without restraint. Similar in design to Starcraft's Zerg, the aliens from the Alien film series, or the Flood from Halo, the Swarm is the primary antagonist in the game.
The Swarm's resistance to damage, speed, and damage output is based heavily upon the difficulty level selected.
Swarm enemies are rarely referred by their names in-game, but these names are gathered from the game's sound files.
Enemies come in several primary forms, listed below:
These large targets aren't too difficult to destroy, but often strike in large numbers. The so-called "foot soldier" of the Swarm, these hand-shaped aliens use claws to inflict damage. Their melee is their only attack, so players should utilize their superior range to eliminate them from afar. When attacking in large numbers, a rifle grenade or other area-of-effect attack will eliminate these in droves. Drones are also no match for a sentry gun.
Boomers are large tripedal aliens, similar in design to the tripod walkers from War of the Worlds. These targets have a single large sac containing gas and an unknown, highly explosive substance. Although they have a melee attack, these targets are most notable for bursting when attacked. If attacked quickly, however, these targets can be destroyed without bursting.
Due to their relatively high damage resistance, however, bursting is quite common. If the boomer bursts, it scatters several large green gobs. After a few seconds, these gobs explode, dealing considerable damage to nearby allies. These explosions are highly damaging and should be avoided at all costs.
The primary flying swarm, these small gnat-like enemies cause little damage, but cause the player's screen to shake and stumble, often leaving a marine vulnerable to attacks by other enemies. Their size and evasiveness in particular makes friendly fire a considerable problem, and their screen-shaking attack only excaberates the matter.
Fortunately, buzzers are extremely weak defensively, and are often best dealt with using melee attacks.
Eggs, Parasites & Harvesters
One of the most annoying enemies in the game, eggs are completely harmless, until they hatch. Upon hatching, they release several parasites. Note that eggs still count as "alive" even after hatching, and must be destroyed in order to count for any objectives. They will not, however, spawn new parasites.
Parasites are small, highly evasive and highly damaging enemies. They are found on their own, inside eggs, and are spawned from downed players if another parasite kills them. Parasites are capable of attaching themselves to a target, and will quickly kill their host if not treated in time. Unlike other "attaching" enemies from Valve's shooters, healing is the only way to remove a parasite once it's attached. Players can also opt for electric armour, but this is an unlockable feature and can preclude the use of other items like welders. Electric armour must be activated (pressing the 1 key) to kill an attached parasite.
Parasites are extremely weak and have the least health out of any of the infected (buzzers are the only comparison health-wise) but their swift nature, damaging attack, and the difficulty of treating them has the community divided on whether these aliens are overpowered or not.
Both eggs and parasites are highly vulnerable to flamethrower attacks. Parasites will die almost instantly upon touching the flames, and eggs will squirm helplessly, leaving themselves open to another attack. On longer levels such as Timor Station, it is best to set all parasites and eggs alight, and then finish them with melee attacks to save flamethrower ammunition.
Keep in mind that medic's mediguns are not as accurate as they are in other, similar games. Players infested with parasites should run towards their medic and stay still so that the medic does not have to run around trying to heal you.
Harvesters are essentially mobile eggs. Harvesters are caterpillar-like and have a bulbous abdomen presumably containing Harvester spawn or eggs. Harvesters will continue to create harvester spawn as long as they are left alive. Unlike parasites and eggs, however, harvesters are not very vulnerable to fire. They cannot, however, spawn and move at the same time, and value their own survival higher than eliminating players. Therefore, it's often best to use the flamethrower to eliminate any harvester spawn a harvester has deployed, and then simply rush the harvester with whatever weapons you have.
Eggs and parasites are first introduced to the players in SynTec Residential and will continue to appear until the end of the campaign at Timor Station. Harvesters will only appear at Timor Station. On difficulties beyond Normal, however, they can appear even at the beginning of a level.
Rangers & Mortarbugs
Rangers are, as their name suggests, the primary ranged unit the Swarm employs. Once stationary, beyond a player's screen, these bugs will shoot large gobs of acidic spit that travels in a straight line until impacting upon a player or a surface. As with most ranged units, however, they are very vulnerable up close. Their range, however, is considerable and often works in conjunction with drones to keep players suppressed.
Players can useto roll under ranger projectiles. Rolling under them (rather than dodging them, like most players do) earns the player the Under the Gun achievement.
Although never referred to as such in-game, Mortarbugs are so called for their modus operandi, a mortar attack which shoots what appears to be seed-like projectiles. After several seconds, these mortars explode, not unlike the bombs which are the result of a boomer bursting. Players can see telltale signs of the bombs about to explode when they "bloom" and start to glow brightly.
Mortarbugs are extremely large and slow, and have no defences up front. However, the game seems to position them in hard-to-reach places, prompting players to stand and attempt to shoot them from a distance; leaving them vulnerable to mortar fire.
Mortarbugs are also referred to as "Artillery bugs" by players.
The instantly recognizable shield bugs are aptly named for their five finger-like appendages up front that, when put together, act as a shield. This shield is virtually impervious to standard attacks. The most common solution to shieldbugs, therefore, is to flank them and hit them in their vulnerable rear half. A good shot to the glowing section in the rear of the shieldbug will put it out of commission easily.
Shieldbugs have a considerably powerful melee attack which should be avoided. The attack will often send a player flying backward, stunning them for a brief period.
Masses of living tissue, possibly organs, are targets found in Timor Station and SynTec Residential. The game's briefing suggests that this biomass is the first step towards hive construction. Regardless, biomass is large and stationary, impervious to normal attacks, they can only be removed by using the flamethrower and, later on with the mining laser. Destroying biomass often open up passage beyond it and calling a large mass of Swarm to attack in retaliation.
Avoid touching biomass, as it results in a small amount of damage.