I'm working on my warrior BP, but this is probably very similar for any task that Trelak offers.
I tried both Dranik Scar and Ruined city of Dranik.
While I agree that Dranik Scar may have more mobs, it's hard to beat ruined city of Dranik (RCoD). Once you learn the circuit in RCoD, it's very easy. Also, if you need to afk and come back every 16 mins for re-pops, the room with 36 spawns in incredible. Also, just up the hall from that room is another 20-30 mobs. It was quite easy to do. Dranik Scar require a lot of running around and the mobs weren't necessarily near eachother. RCoD has bundles and easy rooms to pull.
After grinding to Ally, I went to Trelak and discovered that I needed drop... drops that give faction while grinding for.
Word to the wise.. get your drops first, THEN check your faction. I could have save a bunch of time if I had planned ahead.
Good luck