Source: GW2Wiki (with some changes). Thanks to whoever wrote it
The objective here is to escort Glenna to the inner gates of the stronghold. For most of the part you will have to work in two teams, one Escort group and one Tower group. The Tower group will make it possible for the Escort group to advance and vice versa.
Mastery requirement: Tower group need "Ley Line Gliding"
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Once the encounter is begun, each player will have the option to use their special action key to call Glenna over to their current position. You should assign one person to babysit Glenna. Note that throughout the entire encounter, at least one player is required to stay within 800 range of Glenna in order for her not to become paranoid, and ultimately killed by White Mantle Seekers. Should Glenna be killed by any means throughout the encounter, the entire player squad will instantly be defeated so it's important that the assigned player keeps her out of harms way.
The first obstacle is a lowered drawbridge. There is a trick here that can be used to save some time, once you speak to Glenna there will be some talking and once you all get the special action key you all suicide (/gg, /qq, /ff). This will bypass the bridge mechanic and it will remain up for the rest of the raid lockout.
If you wish to do it normally you will have to call Glenna to the white glowing circle at the base of the drawbridge. When Glenna is called to stand in the circle she will cast a spell to summon a ley-line. Players crossing the bridge will be able to interact with a lever raising the drawbridge. With the drawbridge raised, players can call over Glenna to the gate in order for her to blow it open. Be wary of the explosion as it will down any players caught in its blast, Glenna included. After the door is breached, White Mantle will spawn and must be killed in order to safeguard Glenna. After defeating the first group of White Mantle, the group must split up into two groups (tower group and escort group).
I would however suggest sending only a Mesmer through the Mushroom Cave with the portal skill equipped. Running with 5 players in there can make it tricky for the ones not running first, since it's easy to get caught by the mobs aggroing on the first player. Once you reach the Bouncing Mushroom leading up to the first tower you will make a portal and get the Tower group up next to the mushroom.
It is imperative that all members of the tower group have the Ley Line Gliding mastery. Both groups will need to proceed with coordinated effort in order for this encounter to succeed. The tower group (or just the Mesmer) must head through the Mushroom Tunnel. Note that throughout this tunnel a deadly effect (Toxic Spores) is omnipresent, and only walking through Thicket Waters (requiring the Forsaken Thicket Waters (mastery)) will allow players to survive the entire length of the tunnel. Within the tunnel are also multiple Mushroom foes, that are best ignored. Note that these creatures have a fairly high respawn rate so I'd advice against killing them. It's also easy to get left behind (and caught by the mobs) so make sure you run together.
Once you pass the tunnel part, the group will be able to jump up onto the tower with the use of a Bouncing Mushroom (this is also where the Mesmer portal is placed if you choose that approach). Atop this tower will be multiple White Mantle mobs. Once the number of players on top of the tower outweighs the number of White Mantle soldiers, the tower is considered captured and will cease firing at the other players below. Should all White Mantle soldiers be killed, a new wave will spawn in an attempt to regain control of the tower so it's recommended to keep one alive with one player staying up at all times.
If you choose to have Mesmer with you, having the Mesmer as the staying player is nice as he can make a portal when it's time to go for the next tower, saving you some time.
While the tower group is progressing through the tunnel the escort group will progress forward while escorting Glenna forward. The tower group will encounter multiple, unending, waves of white mantle soldiers and additionally two mines. If a player (or Glenna) steps within the red circle around the mine, the player will instantly be killed. However if players stand within the outer white ring, but outside the red inner ring, the mine becomes active and can be destroyed by any attacks reaching it. Mines will be present throughout the escort and will respawn at random rates.
Depending on either group's speed, the escort team might need to wait just outside of tower range until the tower group has secured their first tower.
The moment a tower is secured, multiple things happen all at once. Of first concern, a Warg Bloodhound will spawn at both the entrance of the Stronghold and at the end. These Wargs are immune to hard Crowd Control effect, but vulnerable to soft Crowd control such has Chilled, Immobilize, Cripple, etc. These Wargs will completely ignore players and go directly for Glenna, should one reach her, a single attack will be sufficient to kill her. As such it is necessary to deal with these Wargs prior to them approaching Glenna.
Additionally, once the tower is secured, the escort group will be able to advance Glenna to the next glowing white circle to summon a new ley line which will both link the first tower to the second, and offer a ley line from the ground up to the first tower. Thanks to these ley lines (or Mesmer portal), it is possible to send (if desired) 4 players of the tower team (given you keep one mob alive in the current tower) to assist the escort group until they are needed back up to take over the next tower.
The entire siege requires all 5 towers to be captured; once the fifth tower is cleared (kill all mobs here), McLeod the Silent will spawn in front of the doors locking off the courtyard.
McLeod the Silent
One of his core mechanics is to teleport to the furthest away player, as such it is not uncommon for him to teleport to the top of the last captured tower. If this happens, the players in the tower should continue their return to the rest of their squad, and simply wait for him to teleport once again.
Something to keep in mind here is that Glenna no longer needs babysitting and you can leave her at the stairs (despite the icon showing above her, you will not wipe when she looses her stacks). I'd also recommend everyone to move to the stairs next to the door and kill the boss there.
There are three main mechanics players need to be aware of while fighting McLeod. His main attack resembles the mesmer's Blurred Frenzy and will apply a very high amount of confusion to players. The second mechanic which this boss introduces occurs when he reaches 75%, 50%, and 25% health where he will disappear and two clones will take his spot. At this point each player in the squad gets assigned a random attunement; either Red or White. Both clones will also have a unique attunement, and can only be damaged by players of that same attunement.
The moment both clones are defeated, McLeod will return to the battlefield, as well as summon a Warg from the nearby gate, which is also why it's nice to all stack here so you can cleave everything down. This Warg will behave identically to those fought during the escort; and must thus be dealt with swiftly before it kills Glenna.
Once McLeod is defeated, players are free to bring Glenna to the final closed gate for her to blow it open with her explosives. Once the door is blown open, the encounter is considered completed; players will have access to the McLeod the Silent's Chest and be able to walk freely throughout the courtyard, the dock below, and the previous path which was used to escort Glenna.
A piece of advice would be to ensure Glennas escape from the final blast, as if she dies the whole thing will bug out and you'll have to reset the instance as the next boss will not spawn. Although you will still be saved to having defeated the Escort event but no loot will be recieved (I learned the hard way).
And ofc, no guide without Lulle Art!
Source: GW2Wiki (with small changes). Thanks to whoever wrote it