complete every Underfoot quest, task and group progression mission and this mount is yours.
60 mercenary tasks
60 partisan tasks
12 group progression missions
the majority of the first 120 tasks (merc/part) can be requested with you and a merc, a few needed a 3rd person in the group. I breezed through the 120 tasks in about a week (100 raid geared Ranger + cleric merc). as for the 12 group missions I can't recall much about them as I did those back when UF was released.
tip: grab as many tasks as you can in each zone to cut down on time spent killing the same mobs a second time, and do the Partisan tasks first as those usually have you running around collecting specific items off of mobs which will also count towards the Mercenary kill tasks for that same zone.
Edited, Aug 4th 2014 9:25am by Zaztik