One must complete Undaunted Dailies. There are also achievements for healing, blocking, etc, in dungeons. You can also buy a drink for your fellow Undaunted in each zone to receive a chunk of XP towards the skill line.
Initial dungeon quest completion, daily undaunted pledges and completing dungeon achievements. For example killing X enemies of this type, speed/no death or any of the nickel and dime ones like avoiding all watchers in ICP.