So, i basically copy+pasted this from a guide i wrote for my guild. We do Blackthorn's often on Wednesdays with about 10-20 people. Here you go.
THE OBJECTIVE
The objective of Blackthorn's Dungeon is to defeat the Lighthouse that spawns in a random "city pod" inside the northern part of the dungeon. The Lighthouse itself starts off invulnerable to attack, until you kill enough Captains. The Captains themselves are invulnerable to attack (but still affected by Paralyze Field), until you kill their surrounding Minions. So you have to kill the Minions, then kill the Captains, then bring the Lighthouse health down to a certain point, at which point the Lighthouse will become invulnerable again and spawn more Captains/Minions. Rinse and repeat 3 times (75% Health, 25% Health, then 0%) until the Lighthouse is destroyed. A new Lighthouse will spawn within another city pod within 10-15 minutes.
GENERAL TIPS
If you're having trouble targeting the Captains due to how many people/pets are attacking them, then hold Ctrl+Shift, right click away the other name tags, until you see a red tag with the name of the Captain. Left click on that tag, hold, and drag to get it's health bar. Double left click while in war mode (Warrior) or target (Caster/Tamer) the health bar to attack the Captain. Otherwise, targeting the Captain can be very frustrating with how many people are attacking it.
All of the Captains and Minions inside Blackthorn's Dungeon are immune to Discordance skill debuff, but have 0 Resisting Spells skill. This leaves them incredibly vulnerable to debuff/disabling spells if you have Eval, and makes Corpse Skin (In Aglo Corp Ylem, Necromancy spell) last a long time against them.
Paralyze (An Ex Por, 5th Circle Magery spell) is useful for paralyzing individual Captains, which is particularly useful for temporarily disabling a Captain you don't want to fight yet as you fight another one, or catching them in an X-Field. Paralyze Field (In Ex Grav, 6th Circle Magery spell) will paralyze any opponent that runs into it (including Captains even when they're yellow [invulnerable]). This can and does save lives, and makes the spawn MUCH easier to handle, Para Field should be cast extensively by all Eval Mages, especially when pushing into a new wave of mobs. The paralyze effect will be broken on any damage taken (including DoTs like Poison, try avoiding that), so do not attack a Captain that you see someone attempting to Paralyze. (An Ex Por in particular).
Do NOT use Energy Fields (In Sanct Grav, 7th Circle Magery spell) except in very specific circumstances i list on particular Captains, as they block guildmates and their pets from moving, which can lead to deaths with improper E-Field placement. Parts of the E-Field can be dispelled with Dispel Field (An Grav, 5th Circle Magery spell).
Mana Vampire (Ort Sanct, 7th Circle Magery spell) is extremely effective against the Captains. Most melee Captains have a limited Mana pool, and so you can deprive them of their use of their Special Moves by Mana Vamping or Wraith Draining them to low Mana. Caster Captains have a virtually unlimited Mana pool, and so are useful as "mana batteries" to recharge your own Mana from with either Mana Vampire or Wraith Draining.
Curse (Des Sanct, 4th Circle Magery spell) and Mass Curse (Vas Des Sanct, 6th Circle Magery spell) can be used to debuff STR/DEX/INT by a %, slightly lowering the victim's damage output.
Sleep (In Zu, Mysticism spell) and Mass Sleep (Vas Zu, Mysticism spell) do not function at all against the Captains unfortunately, they're immune to them.
The melee captains can have their damage output greatly reduced by a Macer's "Stagger" (-60% SSI for 10 secs, can be refreshed) Mastery ability or a Fencer's "Pierce" (Damages 40% of Max Stamina over 10 seconds) Mastery ability. Three Pierces over 30 seconds is enough to reduce any Captain to 0% Stamina, which makes them swing and move extremely slow. Combine Stagger with 0% Stamina, and the Captain's melee damage output is neutered.
All of the Captains and Minions will randomly have either High Resists/Lower Health, or No Resists/High Health. The Captains also take half damage from pets/summons, and have a 20% chance per melee attack of inflicting triple damage to pets/summons. Pet debuffs (such as Rune Corruption/Conductive Blast/Inferno/Armor Pierce) function fine against them though, which is why i prefer to bring debuff pets to this hunt