Primary
You begin the mission facing a group of Pinesouls and Ice Imps. Beware, because near them there will be a group of Mursaat. They are the floating beings with golden outfits and black wings coming out of their backs. Avoid them at all cost. All Mursaat (this includes spell casters as well as Jade Armors and Jade Bows) have a powerful skill called Spectral Agony. For now, this power will wipe out your entire party in a matter of seconds. It will massively drain away your health. (Though it may be possible to survive limited, brief encounters if you have a couple of healers and use the Heroes that were introduced in the Nightfall campaign.)

Sneak around the Mursaat and head left. You will come across an area being pounded by siege weapons and several groups of Stone Summit. Kill them and pass the siege area. You will see a bridge and a way down onto an icy path (where the Stone Summit have set up their siege weapons). Go onto the ice and destroy the group protecting the first siege weapon and then the second. Fire the second weapon (point A) and you will see a cutscene where a herd of Yaks rampages through the White Mantle encampment and helps destroy the siege weapons and forces for you.

Now, go back up and cross the bridge into the White Mantle camp. Immediately up the hill will be a gate guarded by Jade Armors. Avoid them. If you continue to the left of that gate, you will find a path around to another door guarded by the less imposing White Mantle. Kill the patrols, grab a keg from the nearby powder station (point B) and blow the door away.

You will enter an ice cave. If you keep to the left side of the cave, you will be able to go around all the Mursaat groups (facing a few White Mantle groups) and out into an area that has a circular path with a pit in the middle. Evennia and Saidra are opposite to you on this circle (point C). You can go either left or right around the pit, but there is one patrol of Mursaat to avoid, which is easier if you go left. Then there is a patrol of White Mantle Savants and Sycophants, which you can take out. There you will find a cave that will take you around the left side of the circle to very near the cell. There is another patrol of Mursaat to dodge here. Get a keg from the nearby powder station (point D) and blow the cell door.

A cutscene will show you that Saidra sacrificed herself for you. Now you will need to blow a gate, then run Evennia to the ship. Follow the red path on the map to the ice ship (point E).

Notes: Despite her assertion that you should stop for nothing, Evennia will stop and engage any enemy that crosses her path. If you wish to run, keep an eye on her; if she attacks a group of enemies, you will have to attack too because otherwise she will die and you will fail the mission.

When you're near the ship (point E) there will be some Stone Summit (with a boss) engaging the dwarves that run the ship. They usually wipe out the crew if you move too slowly, so as soon as you can, engage and destroy them.

The Mursaat following you (a large host that will emerge after you come out of a small cave past Evennia's cell) are very slow. You do not need to panic: engaging every Stone Summit group on the way does little to worsen your chances of making it. The Mursaat will be wiped out by catapult fire near the end of the path to the boat.

Tip 1: You can fetch Evennia and then let the Mursaat get ahead of you. They'll slice through the Stone Summit and then get themselves killed by siege weapon fire. Follow them at a distance, and once they've done your work for you and gotten killed, proceed as described above. Note: It has happened that the Mursaat have got stuck on the northernmost part of the path, before reaching the area where they are killed. If this happens, try sending a party member to try to lure them ahead or use heroes/pets to engage them (if your party can quickly take them down -- see tip 3 below).

Please bear in mind this doesn't always work, they can sometimes make it through and stop around the boss. If your party includes Heroes, you can attempt to clear a few Mursaat (per Tip 2, below). If you have only characters and henchmen (without infused armor), you will almost certainly not be able to continue the mission.

Tip 2: The Heroes introduced with the Nightfall campaign give you added tools to take on one or two Mursaat and live to tell the tale. Hero armor is infused from when you first gain them, giving Heroes increased survivability that your character lacks if your armor is not yet infused. The Mursaat will generally use their Spectral Agony skill early in any fight, and it takes a long time to recharge. If Heroes and/or pets first aggro the Mursaat, they will likely expend their first Spectral Agony blast on those survivable or expendible targets. While you should still try to avoid them, don't panic if your party accidentally aggros one or two Mursaat: If your character and enough of the party survives the first onslaught, a reasonably balanced, competent party should be able to take a couple of Mursaat down quickly before they can use Spectral Agony on your vulnerable character(s) or henchmen. This also means that if you choose to let the large party of Mursaat go first (Tip 2, above), a party with Heroes has a good chance of being able to clear the path of any individual Mursaat who get "stuck" along the the way. (You still haven't got a chance against a large party, but you can pick off stuck individuals who are left behind -- especially if their Spectral Agony skill hasn't yet recharged after they used it on another enemy.)

Bonus
The bonus in this mission is rather straightforward, but requires that you fight at least two groups of Mursaat. However at this point, you probably do not have infused armor (this happens next mission normally) and as such you cannot stand up to the Mursaat and their Spectral Agony skill. It is possible, although time consuming, to infuse your armor before this mission by travelling all the way up to Mineral Springs to find the seer who can perform an infusion. (Alternatively, you can travel to the Iron Mines of Moladune and do that mission first (out of sequence) as most consider this to be the easiest method of infusion.)

Bring two powder kegs into the cave.

The bonus itself is to rescue an imprisoned dwarf. There are two parts to this:

1. The prisoner, Rornak Stonesledge, is located in a cell that is inside the ice cave you took to get to the circular path. Before exiting the cave, you will see a path in the cave to the right (green path on map). This path is guarded by Jade Armors. Fight them, and then fight the Mursaat spell-casters after them. You will then get to a locked door (point 2); blow it open with a powder keg (powder stations at point B and point D). This door leads to a snowy path guarded by White Mantle; at the end you will find a small cell with Rornak inside. You will need another powder keg to blow open the cell door.

2. There is a second dwarf, Hamdor Grandaxe, who is looking for the prisoner. He can be found after rescuing Evennia. After you blow the gate to flee, you will enter a short ice cave. After you come out, you will see a path and a large number of Mursaat coming from your right. Evennia will ask you to run; ignore her advice and wait at the exit from the caves. The group of Mursaat will pass from the right to the left. Now go where they came from and you should see Hamdor there (point 3).
Now you can actually go about doing this bonus in either order:

Free Rornak and he'll ask you to speak to Hamdor.
Speak to Hamdor; he'll ask you to look for Rornak. Go back and free Rornak and he'll thank you and you'll get the bonus.
Either way, you will notice that when you come out of the cave to speak to Hamdor, the large host of Mursaat will appear and will be ahead of you on your way to the boat. This is not as bleak as it seems. You can just follow them slowly, and the catapults of the Summit will take them out before they get to the boat. If you don't want to follow the Mursaat, but want to talk to Hamdor, you might let them lead your group until they get to point 4, where they will engage some Stone Summit. You can quickly take the other route (the green one) to get ahead of them.

Note: Either way you do the bonus you need 2 Dwarven Kegs to blast through the doors. If you are in a group of people, make sure two people take them. If you are with henchmen, because the mission is easily done with them, be warned, there is a lot a to-and-froing so for bonus you may be better off taking at least a couple of human players.

Warning: It's possible the White Mantle Zealots who patrol near Rornak's cell get too close to him. He will sometimes leave the cell (even though the door is closed) and start firing his bow at the enemies. This allows them to possibly kill him, thereby failing the bonus. Try to take out the Mantle close to the locked door to avoid this bug.