The Frost Gate (mission)

This mission can be easily solo'd with henchmen. The bonus however is probably too hard (see below).

Primary
Destroy the Catapults, open the gate The catapults are easy to find since destroying each opens a gate which allows you to travel further south. You then have to clear an area and put a "gear" into 3 winches which together open the frost gate and complete the mission. Not mentioned is that you need to put the "gear" into another mechanism first to get to the area with the 3 winches.

Bonus
Find and return the Plans to the Dwarf The bonus mission is triggered by rescuing the dwarf located in the dryder caves. He'll ask you to help him retrieve some secret plans from the Stone Summit dwarves nearby. The secret path he mentions is roughly straight north from the nearby besieged bridge; once you've found that, simply follow the path around to take over and fire the derelict ballista, then go back and head across the bridge. You'll need to carry the plans back but there are no ambushes.

The cleanest way to complete the bonus is to avoid the dwarf until you've cleared out both the area across the bridge, and the path leading to the derelict ballista. Then talk to him, walk him down, park him at the ballista, and go get the plans. Skills that do area effect damage, or that reduce Health and attack speed (Necromancer and Mesmer skills, for example) can make the fight with the dwarves guarding the plans a lot easier. Also remember to defeat any Dolyak Rider or similar "healer"-type enemy first, or you could be at it a long time.