Sailen, which located in the region called The Great Desert, has two dungeons: Sailen: The Lower City (GSL) and Sailen: The Palace (SPL). For the most part, neither of these dungeons should be attempted, especially with pick up groups (pugs), until you are well geared. They were designed and implemented after players had the first three dungeons (RKL, KPL, ETL) for awhile so the expectation was not that players would attempt these in blue gear and certainly not in green. It is possible to complete them in inferior gear (less than 4 epic pieces) but when you run a dungeon with insufficient gear you are relying on mechanics (knowledge of the dungeon) which is difficult in pick up groups and for others to make up for your lack. In my opinion, DPS is probably the factor you can skimp on the least as without sufficient DPS you will take incredible amounts of damage that no tank can survive and no healer can heal through.
As you begin this dungeon you’ll see two large minotaurs. These are the most difficult monsters in SPL and GSL. They do a spin move and deal alot of damage to the entire team. It’s best if your team focuses on one at a time and uses stuns wisely. As you follow the path laid out before you make sure someone is gathering the Resonance Mixtures that drop which the tank will need to use at key moments on the first boss. Also be aware that there are some monsters (Hunters) who will shoot you with arrows and not come when you pull the groups. You’ll need to make sure they don’t target other members of your team by pulling the group on top of them and grabbing aggro.
The first boss is a giant looking beatle called Titehelm. It’s helpful to pull him to one side of the room (I usually pull him to the far right) for the scorpions. During the battle Scorpions will be summoned and come at you. They explode when they reach the target, so by moving Titehelm to one side of the room, the DPS can see them clearly and kill them before they do any damage. A ranged DPS is ideal for this role. There is no specific moment to use your defensive skills, so just use them when the healer is having a tough time or your health is low.
As you continue to battle him, be aware of the what Titehelm is saying in the red words of your dialogue box. He will say a number of things, but when and only when he says “When you feel the earth shaking, it is time to say goodbye”, the tank needs to use one of the Resonance Mixtures spoken about earlier. Place the Resonance Mixture in your action bar so you can get to it as quickly as possible. If you don’t use it quickly the entire team will die! This will occur three times during the battle so make sure you have three resonance mixtures!
One of the more difficult pulls is after Titehelm and involves 3 different Minotaurs. Most groups will pull them from afar (use an arrow) so that the team misses their first spin attack. Just make sure melee DPS understands this and that they don’t charge in until after you establish aggro. This is a good moment to use your defensive moves to keep alive! Also at this point is a portal that you can use to summon a strange creature from another reality. This is for a quest given outside the dungeon and most teams skip it. After this pull, there’s a fun tunnel to run through. Most teams run all the way through it and stop at the Minotaur on the other side because the two goblin groups aren’t not difficult to take at the same time. After that you can run straight to the final boss. There are a number of pulls you can skip in this dungeon so if its your first time let someone else lead the way.
Kha-Anata is the final boss of GSL. You’ll notice that he has 3 vines that are around him. These are probably the key to this battle and the reason why you need good DPS. Attack the first vine on the right and kill it. Then move to the second vine behind him (counterclockwise) and then the final vine. It’s helpful to use any stuns you may have to help prevent the vines from doing too much damage. If you downed the vines fast enough then you’ll have some time to hit Kha-Anata a few times. You’ll notice that he isn’t taking any damage from the 0′s over his head but it’s good to go ahead and get aggro.
As Kha-Anata begins to actually battle you, move him over to the water on the side where you killed the last vine. Some groups 
prefer that you move him all the way into the water completely. He’ll summon other similar looking creatures during the battle, but if you have your team on this side then they won’t damage you and can be ignored. Occassionally he’ll cast a “powerful spell in front” so make sure he’s not facing the group when that happens. Probably most importantly is that you’ll read text that warns you of an attacking vine. The DPS need to take this vine down as quickly as possible.
Unless your DPS is amazing, the tank will need to switch over to the vine as well, again utilizing any stuns you may have. The best place to use your defensive moves is near the end of a vines death, especially if they are taking a while to get down. When you get down to the last bit of health Kha-Anata will begin to glow. At that point you won’t have to worry about any more monsters or vines being summoned and you can be fairly confident that you’ll kill him. In addition to the loot on Kha-Anata (don’t forget your soul signet!) there is also a chest that appears. The chest has the possibility of dropping some epic loot, but that is certainly not always the case!
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Thanks Samuel for this guide.