The best way (simple and time saving) to Duel Faction farm now is to use the NEW TOMB B/P ranger team build.
The B/P Team build is exactly like NEW TOMB B/P ranger team with slight modification. There will be no MM in this team however substitute by an A.Echo/SS Necro instead.
The BA Duel FF barrage/pet team consists of:
5 Barrage/Pet Rangers R/X
(BARRAGE, Savage Shot, Distracting Shot, Throw Dirt , SPIRIT [FW,Winnowing or Frozen soil] , Storm chaser , Charm Animal, Comfort Animal/Rez)
1 monk Mo/N (Healings with OoB for energy management)
1 SS necro N/Me (SS, Awaken, Archane Echo, Desecrete Enchantment, Suffering ....)
1 Order of Vampire Necro N/Mo(Hp at 105, with mending & Protective spirit, awaken of blood, blood renewal, Heal Party)
The tactic is pretty simple - Summon those spirits i.e. winnowing, frozen soil, fav.winds outside the dueling zone. When every Spirit is up, go in and start attacking with Barrage (Rangers need to bring down duelist healer a.s.a.p)
Order necro should always stay outside of the battle field while spamming OoV or HP, as OoV or HP will affect every teammates within the minimap.
For more information regarding build, here is the gw-wiki link
http://gw.gamewikis.org/wiki/Team_-_...8Tomb_Ruins%29
It works for me, and it took around 2mins for a FF run.
pm me if you need a Order Necro, i'm pretty good at it, cuz i'm lazy..
Try it and have fun