You get undaunted xp by completing the daily pledges and by completing the group and veteran dungeon achievements.
The quickest way to get it done is to find 3 like minded (and experienced) players and run each normal dungeon one after the other aiming to get all three achievements per dungeon in each run (boss kills, speed run and no deaths). This will net you 30xp per dungeon and a bonus amount of xp when you complete all of them iirc. You can then go on and hunt the achievements for the veteran dungeons as well.
Completing the daily vet gold pledge will give you 20xp and the silver normal pledge 10xp regardless of whether you've completed that dungeon before. You can also complete the 'This One's on Me' achievement for a bonus 30xp - im sure you can google all the locations or download the Destinations addon to help get it done.
The reality is though it can't be done in one day!