Ferghus How to get that quest?
There's a lot of steps. First, you need to do to do the quests associated with Lisa (
Overlord M405 Cannon and
Repairing Overlord M405 Cannon), if you haven't already done them. On
ZedTed's guide, it's listed as the first two quests under the Pirate's Bane section. Then you need to need to do the questline for the South Gate Camp (which despite its name, is in the north).
In the fort is a dude in armor with a quest marker over his head. First he'll ask you to look for 3 dudes. After you finish the quest, wait a couple days. He'll have a new quest for you to follow him to check out some ruined houses. After that, he'll recommend that you do a quest by the area's Inn/Guild. I don't know if you actually need to do it, but by the time you get back, he'll have a quest for you: Bring Lisa and her cannon. You can take the carriage to Noer (unlocked after the first quest in the area) and back at this point, costing a little over 4k Trade Points each way.
Note: This is the point of no return. Once you ask Lisa for help, there's no way to abandon the quest manually, and she'll be following you for up to two weeks before the mission automatically fails. She'll follow you until she gets kidnapped, at which point you have
3 days to save her. After that, she'll resume following you until you take her back to Noer.
Talk to Lisa at her usual spot at night. When you ask for her help, she'll ask to be escorted to the fort where you did the Overlord Cannon quests. When you finish talking to the guy at the top, Lisa will ask to be escorted to to South Gate Camp. After some dialogue with the guy in the armor, you'll be asked to work the cannon a short distance away. After fending off a horde, they'll inform you that Lisa got kidnapped and taken to one of the nests. That is when you get the quest to save her.
P.S. Lisa will also be incapacitated following the quest. She appears back at her usual spot after 3-4 days.