The League Will Lose

Had the same issue. My guess is that this quest is only possible to complete when the Horde controls Arathi because that's when the League of Arathor mobs spawn at the location indicated by the map. All the other quest mobs for this quest giver are constant spawns, though their spawn points move depending on who controls the zone.

I've got the quest but I am unable to find any relevant mobs at the area.
I picked up the quest 2 weeks ago. Maybe its related to that?

So for horde, there's a big area way out east of refuge point. I found it easy to go down and across the southern quests and back up the right side to The League will Lose last. Got lots of rares along the way too. (Ok I did see a vendor in refuge point, jump in thinking it was horde, died immediately, but set that aside.) Edit: forgot we could fly here too. That'd be another mishap, but I got through all the opening quests!

The Arathor npcs are smarter than most. Best to just kite one guard at a time off the perimeter to start. If you're melee, save something big damage to kill them fast when they get to 10% or they run off - sometimes to a healer or more npcs. Once you kite a hole in the perimeter, go in and try doing one at a time as long as you can. Once packs get on you, find and kill the clothie healers waving their glowing hands around. They'll keep everyone alive while you die with 0/20 of your quest kills if you're solo and don't kill the healers first thing. After the healers, kill the mages, kill the small guards, and deal with the elite horseman guy 1-1. He'll be kicking you around but hopefully you killed everything around for a bit. If you're too slow, you'll find yourself with a 10% horseman and 15 more mobs with mages and healers working. If you make a run for it, 2 or so npcs will chase you way far away, but not all of them. Then if you're 19/20... run back and kite a guard to finish the quest... ilevel 347 vengeance demon hunter alone. BM hunters and other classes might have it easier or harder. Good luck! (Oh yeah, those trolls to the south - don't pull too many of those or you'll be polymorphed and netted into oblivion. That's another quest mishap of mine.)