Phantom

The Phantom is a type of Cerberus unit encountered in Mass Effect 3. Foes and allies alike have described the Cerberus Phantom as "Goddamn ninjas on speed!" with various levels of admiration or dismay.

Capabilities
Offensive

Phantoms are deadly in melee combat. A Phantom's most feared weapon is its ability to kill its target in melee without any chance of getting revived. They also possess a powerful semi-automatic projectile-based weapon, shot from the palm of their right gauntlet.

Defensive

While they are protected by biotic barriers, Phantoms rely on their agility to evade enemy fire. They can quickly sidestep, roll, or leap away from attacks, disorienting the player. If endangered, Phantoms will also activate cloaks and take cover for a short period. They can recharge their barriers and their health over time.

Tactics
General Tactics
 * Phantoms are almost as, if not more dangerous than Atlas mechs in terms of the damage they can inflict. The safest way to deal with them is from a generous distance. While they possess a ranged weapon, they only employ it while relatively close. At longer ranges, they attempt to take cover or activate their cloak instead.
 * Phantoms can instantly kill a character in melee, but only if their target is stationary. Moving around denies them their instant-kill attack, but they can still kill you with sword and gauntlet shots.
 * If you want to leave her close range, fire any projectile power with a travel time, such as Warp, Singularity, Throw, Incinerate or Cryo Blast. The Phantom will attempt to dodge the attack, which delays her attacks for about two seconds. You can use this opportunity to run away.
 * Since the Phantom is able to dodge most projectile powers, it not recommended to use these powers with the intention of damaging the Phantom. It has also been noticed that Phantom can use warp to replenish it's barrier. However, if you have a very fast cooldown, you could spam warp and other powers and damage it as the Phantom will not dodge the second or the third attack.
 * If a Phantom cloaks, try to focus weapons fire on it as long as possible to disrupt the cloak, otherwise it will be completely invisible. At very close range, the cloak is as visible as an infiltrator employing Tactical Cloak.
 * Particularly in the later rounds of multiplayer matches, Phantoms are prone to pairing up with a Nemesis. Because the Nemesis has no close range attacks, and is best dealt with by melee attacks or shotgun blast, it serves as a very effective lure for the Phantom. Keep this in mind when watching Nemeses move around the battlefield; if they remain in one corner for an extended period, they are likely being used as bait by a nearby Phantom.
 * Once their barriers are down, biotic powers can be employed to incapacitate them (such as Singularity).
 * Phantoms' seem to be immune to the Cryo Blast complete freeze effect, even when their barriers are down. They can be slowed, but not frozen completely.
 * Stasis can incapacitate Phantoms even with their barriers up.
 * Phantoms regularly take cover while closing distance towards targets. Due to the design of cover, their heads are often slightly exposed, making it easier to line up headshots. This is particularly useful if you have a high powered weapon that can instant kill a Phantom with a single shot.
 * It is possible to shoot the Phantom's sword from its grasp, making the Phantom vulnerable to attacks without the worry of being attacked with its fatal melee.
 * Using melee attacks against the Phantom is risky, as she is capable of killing almost any character faster than they can kill her while trading blows. Combining melee and any staggering ability such as Concussive Shot can turn the tide.
 * Damage-over-time powers, like Warp or Incinerate, can be very useful in taking down Phantoms. While they may not kill the Phantom outright, when the Phantom hit with the power cloaks, the power will still do damage. Then, when the Phantom can be spotted through her cloak from the damage the power is still doing, she may give you the chance to shoot and kill her.
 * Rolling backward is highly recommended if a Phantom attacks with her sword. If successful, the player not only avoids the attack but leaves the Phantom momentarily vulnerable to a counterattack as she recovers from her sword swing.  Naturally, this tactic works especially well if the player has a shotgun.

Class/Character-Specific Tactics
 * Phantoms are fortified well with a biotic barrier; however, high-ranked Energy Drain or Overload can take down the Phantom's barrier in one use, which will leave her health open for any attack. A properly leveled Engineer can capitalize on the Phantom's cloak-and-cover defense by using either of these techniques, followed by Incinerate to hit around cover. This is a good way to keep them disoriented and on the run.
 * The Infiltrator is capable of using the Phantom's own tactics against them, with a combination of tactical cloak and melee attacks for massive damage or possibly one hit kills if handled carefully. With enough melee damage upgrades, they are capable of killing Phantoms with a single heavy melee while cloaked. This is a very risky attack, as missing or failing to kill the Phantom will leave you very vulnerable and easily killed.
 * Using a shotgun as an Infiltrator is another viable option, as it lets you deal considerable damage coming out of tactical cloak while still being mobile enough to strafe or backpedal away from the Phantom's counterattack. By using the cloak in short bursts, you can minimize recharge time and continue to use the cloak's damage bonus, or give yourself time to evade and reload without interruption.
 * The Vanguard can use Charge to disorient the Phantom, and follow it up with shotgun blasts or Nova. However, this is a very risky tactic which can be fatal if not well executed. With a sufficiently low Charge cooldown, a Vanguard can continuously charge the Phantom, effectively incapacitating them.
 * A slight variation on this method is to charge the phantom, disorientating them, offload whatever weapon you have into them while retreating (the back command should be active while in the charge). As long as the charge is successful there should be no problem in performing this multiple times to defeat the phantom.  There is a small risk that the charge may fail however, in which case, run, if you value your pixilated skin.
 * Another Charge variation is to have a squadmate use Warp on the Phantom, then charge. On Normal difficulty, at least, the resulting biotic detonation will completely remove the Phantom's barrier and about half its health, as well as leaving it staggered and easy prey for a final shotgun blast.
 * In multiplayer, the krogan class is capable of killing a Phantom quickly enough with their heavy melee attacks, however, be cautious that the timing of the attack is right, otherwise the Phantom may deal just as much damage if given the chance.
 * Asari Adepts and Vanguards can easily deal with them with their Stasis.
 * While difficult, a Soldier can, with good aim, use the M-98 Widow to shatter the barriers of a Phantom, then utilise Adrenaline Rush since it replenishes ammunition in the "Clip" and fire a second shot in quick succession. Provided your aim is true, a second shot can critically injure, or even kill a Phantom. This is best employed while the Phantom is in cover.

Trivia
If you do not complete the Grissom Academy: Emergency Evacuation mission in time (either completing three other major missions first or completing Priority: The Citadel II, whichever comes first), then Jack will appear as a Phantom when you raid Cerberus Headquarters. She is no different from any other Phantom and must be killed normally.