You will need to bring a detect invisibility potion (or equipment). Two potions if you don't want to rush the demon king. You will also need a serpent's mask and a way to fly one person.
Any level player can enter SL and help with ghostly hounds, but skatha's will slay anyone under level 44.
In addition, most equipment from SL requires level 44+.
The entrance to SL can be found 2wn2wn4wdene2nwswde from xroads. This will put you 1 west of barney, who is agg.
Go 1e and kill barney. The player who lands the killing blow will be thrown 1w of barney and will need to go 1e again to rejoin the group.
Go 2sw, which will place you 1e of the area of ghostly hounds. You will need detect invisibility to see them. Throw/shoot hounds and move w. Erin is 1n from here and is not agg by default. It is generally preferred to kill Erin last, as someone may need to do the SL2 entrance rights quest and may not speak up until the end of the run. Continue throwing/shooting hounds while moving s, s, n, w.
This will put you 1e of a sea serpent. Have a person who is not flying and is not leading the group "fetch" the serpent. This entails going west and waiting until the serpent attacks you for at least one round, then fleeing back up to the group. After a few moments, the serpent will rush the group.
After killing the first serpent, have everyone leap twice, putting the group 1w of a second serpent. Again, have someone "fetch".
Note: Be sure the group understands that the keyword for the next mob is skatha's not skatha. The latter will attack Skatha instead of the skatha's the group is throwing/shooting into the room.
Throw/shoot skatha's while moving n, w, s, e, until you're back at the room with Skatha and no more skatha's remain.
Throw/shoot the horse and move south to kill the shade.
After killing the skatha's, you can kill Skatha. Or save her for the end of the run and end up forgetting about her.
After killing the shade, move 4n to the room with the three witches. Kill them all and one corpse will contain a key. Have one member of the group who is flying and not being followed by anyone, wear the serpent mask and go all east to unlock the chest and get the new key from the chest. Bring this key back to the leader.
From the room with the witches, move s, w. Throw/shoot son while moving n, w, n, n, n, e, s, s, s. This places you 1w of Calatin's room. Make sure everyone has protection vs. fire, and throw/shoot Calatin.
Move w3nw3sesw from Calatin's room back to the main hallway. Throw/shoot sidhe guards while moving w, w, w, w, w, w. This will put you 1e of the demon king's room.
In order to throw/shoot the demon king out of his room, you will need detect invisibility. Otherwise, you can just rush into the room and he will attack someone in the group and become visible. The demon king goes through three phases. After getting to a certain health threshold, he will turn into a different mob, and again after hitting another hp threshold. Each phase will put the mob back at full health. If you can keep the mob sitting on the ground, it will not go to the next phase and will just die. It is recommended to use protection vs. fire for this fight.
After killing the demon king, you will find Midir. UNLOCK CHAINS to get a whois and allow him to give SL2 entry rights. If someone needs SL2 entry rights they need to do the following:
This will give you a whois and make it so you don't lose all of your health when you use the SL2 portal.
From the demon king's room, move 8e. This will put you 1n of the fallen sidhe room. After entering this room, waves of sidhe will spawn including spectral sidhe, fallen sidhe, skeletal sidhe, and the fallen sidhe king.
To leave, simply move ne2s, leap twice, ene2n back to Ferdia (Barney may have respawned and be 1n from here). Then 2nsws4e2nws2w back to Taran Crossroads.