The Great Northern Wall (mission)
This mission can be easily solo'd if you take the healer, fighter, and mage henchmen with you.
PRIMARY: Observe the Charr camp.
The primary goal is pretty easy to achieve. Simply follow the path northwards, working your way through the devourers. When you run into the Charr boss and his friends, you're almost to the second cutscene. After watching it, it's a simple mad-dash back to the captain. Note that only one party member has to reach the captain; if for some reason you get held up and the enemy forces get too close, someone can try and grab their attention for a few seconds. They'll probably die, but it'll allow the rest of the group some breathing room.
BONUS: Find the spirit's armor.
The spirit's armor is located in the wreckages you find scattered about the second half (once you've passed the lever door) of the map. There are four pieces, and the spirit is located in the northeast part of the second area (the Charr overlook would be the third area; if you're at the Charr boss, you went too far). This may be slightly bugged; it seems best to collect all four armor pieces first (on one character) and then talk to the spirit.