There are two areas where these mobs spawn. The first (and most popular) are the three enclosed camps near the river, east of Skulk Rock. The camps have two Soothsayers inside them, and are guarded by about six Scalpers, two of which patrol all the way around the camp. As there are only three camps, one run through all three will only net you a little over half the mobs you need.

The other area is the Altar of Zuul. This is the same place where you get the sacred mallet needed in Zul'Farrak. All around the altar (without going up the steps) you can find scalpers and soothsayers. There are also axe-throwers and wolf pups though, so watch for wandering mobs.

If you clean out the three camps first, then kill all the soothsayers/scalpers near the altar, you can go back to the camps to get the last few kills you need. Things should have respawned by then.

While you're going through the camps, be sure to grab the quest from the blacksmith in Revantusk Village. You'll likely find his tools in one of the camps.

This is one of those quests where you need to kill more of one mob than another. In either of these areas, there are spawn ""points"" where one of up to 3 kinds of mobs will spawn. Obviously you're going to kill the lower needed mobs before you finish the higher (especially if you're doing this quest at The Altar).

The trick:
Just kill everything.

With the exception of the main path up the Altar, the spawn points will birth Axe Throwers (a big pain cuz you need zero of them) Soothsayers (the lower number needed) and Scalpers.

People make the mistake of skipping what they don't need and going around the others. Here's the problem: Let's say there are 30 spawn points for the area. 15 of them are soothsayer and 10 are axe throwers and 5 are scalpers. If you kill the 5 scalpers, chances are only 2 of them will be scalpers next time. This explains why you sometimes see areas with all the same mobs.

Sooner or later (when no one is in the area) the game will reset makeing the area ""even steven"" but that's too long to wait. Also, walking around for 20 min trying to find the mob you need is killing your xp per hour. Also, more spawn points means a better chance of a rare showing up. The altar area has one.

This is why it's better to just kill them all. If you go ahead and kill axe trowers or extra soothsayers, there is a chance they may birth a scalper (for instance) next time. Plus you gain xp and chances are you'll not agro as many adds when they try to ""run in fear"".

I call this Blizzard game quest tactic ""area killing quests"" if you don't like grinding, I suggest you stear clear of this type of quest. But, be aware, 80% of the games quests are like this.