Guardian (enemy)

Principal Guardian rushing in to attack

Guardians are the protectors of The Vault. They have alien body design, use alien weapons technology and use extremely tough shields to protect their fairly weak bodies underneath. Primarily encountered late in the game, Guardians will often be found fighting against the forces of the Crimson Lance.

Involvement

The Guardians are beings left on Pandora by the Eridian race to protect important Eridian artifacts and locations. One such location is the mine underneath the Dahl Headlands, in which three Guardians (one spectral and two wraiths) have been left as protectors for an Eridian Artifact. Another, more prominent location is the Eridian Promontory where the Guardians have been set up in defensive positions covering the entire approach to the Vault.

Guardians seem to be almost robotic in nature, though they have many organic parts and bleed blue blood. It is unknown if the Guardians are Eridians chosen to guard their valuables, or created by the Eridians for that same purpose.

Further speculations about their origins can be found on the Talk Page

Tactics

• Shock damage is very useful for stripping away a Guardian's powerful shield. Once the shield has been depleted the Guardian itself is not particularly robust and will succumb quickly to subsequent damage. Eridian weapons work just as well as shock. If you lack any of these weapons, shoot for the head.
• The Hunter talent Trespass can be the bane of the Guardians once it has been advanced to its maximum level. It will allow bullets to bypass a Guardian's formidable shield to apply the weapon's full force to the relatively low life of the Guardian.
• The unusual pattern, speed of their movement and slender bodies can make Guardians difficult to target effectively, so using an automatic weapon or a shotgun can be useful in many cases.
• Their heads hang in front of their chests and are quite small so it can be difficult to achieve critical hits on them from a distance, however with the head hanging where it is it can be quite easy to score accidental critical hits from generally targeted body shots when the Guardian faces towards a shooter.

Guardian Types

The Dahl Headlands

Guardian Spectre - A Guardian left to guard an Eridian Artifact entombed under The Dahl Headlands. The Guardian Spectre has a peculiar spectral appearance that sets it apart from the more common Guardians found later in Borderlands. 

Guardian Wraith - A Guardian left to guard an Eridian Artifact entombed under The Dahl Headlands. Guardian Wraiths have a peculiar spectral appearance that sets them apart from the more common Guardians found later in Borderlands.

The Descent and Eridian Promontory

Arch Guardian - A melee oriented, quick moving Guardian that uses energy blades on their wrists to deal damage.

Sera Guardian - The only kind of flying Guardian, they attack from impossibly long range using either a charged stream of energy or several energy blasts similar to the close range attack of the Arch Guardian.

Principal Guardian - A melee oriented, quick moving Guardian that uses energy blades on their wrists to deal damage. They usually come in small packs and are sometimes paired with a larger Badass Guardian.

Badass Guardian - A much larger, more powerful version of a Principal Guardian, also using melee attacks primarily to do damage. These are easily distinguished from other guardians by their larger size and slightly different glow to their armor, and a larger protrusion on their backs.

Notes

• Guardians are hostile to Crimson Lance units, and when they appear together in Crimson Enclave, The Descent and Eridian Promontory, they will fight each other. This can be used tactically by lending assistance to the weaker side, or players can alternatively watch for opportunities to snatch a kill in order to gain the substantial experience rewards.
• Guardians grant very high XP rewards in comparison to their difficulty, making them ideal to use for experience point farming. 
• If the melee Guardians are unable to reach the player, they will start firing plasma bolts onto the player instead until the player moves.

Trivia

• There are references to the Christian Hierarchy of Angels with the enemies guarding the vault: Sera Guardians - short for seraphim, Arch Guardians - short for archangels, Principal Guardians - short for Principalities. The angels alluded to in this naming scheme are the guards of the Christian God's throne (Seraphim) and his foot-soldiers (Principalities, Archangels).
• Guardians are the only units to show their hits on the player's screen, even when shields are on. When attacked by a plasma bolt, the splattered plasma appears on the player's screen. When slashed at with energy blades, the player's screen either gets one slash horizantally, or two slashes horizantally/vertically.