Darksiders

this method involves no serpant holes and is alot quicker then all previous methods posted above, go to the crossroads and kill all the enemies on horseback, after this rather then entering the serpant hole go to the library which was previously guarded by the big troll, enter here kill the enemies and then walk back out into the streets all enemies will have respawned and makes killing on horseback alot quicker!!!!

Go to the crossroads, kill everything there on horseback then SAVE your game, reload it, and do it again. Stupid to run to a serpent hole again and again. It only takes a few seconds to reload your game and everything will have respawned.

Horseman -
Kill 150 demons from horse back.

If you dont have this achievement before you go towards the final boss, there are a couple of good places for this.

The Ashlands, there is a rideable passage in the North North East corner that goes between the ashlands and the Arena(just before Stygian's area). This has clusters of about 5-8mobs, ride through killing each group, there was about 4 groups. For some reason these groups tend to respawn by the time you get to the end of the area, just turn around and ride back through.

The Crossroads/Scalding Gallows.
Goto the Crossroads, mount up and kill everything in this area, should be around 20ish mobs. Use the serpent hole and goto Scalding gallows, kill everything here. Once everything is dead go back to the Crossroads and keep repeating till done.

This one is called Horseman, to unlock it you must kill 150 enemies on horseback.

When you first get Ruin in the arena, and before the Stygian fight you will rack up about 30 to 50 kills give or take. After that the best place hands down to get kills is the Ashland trying to travel back to the dry road. You'll run into groups of 5+ enemies a handful of times.

Another couple of good places are the Choking Grounds and Scalping Gallows. Though there won't be nearly as many enemies they will help build up kills

Best way to do this in my opinion!

To the north of the area called The Ashlands, there is a pool of water surrounded by a wooden structure, as well as a chest containing 500 souls. Every time you jump into the pool of water, two horseman enemies will spawn on the plains in front of you. Now, You can jump into the pool of water repeatedly, meaning that you can spawn as many enemies as you want ( though i recommend around 20 or so, on the basis that my xbox was starting to get a little buggy...) meaning an easy farming session, and the 150 kills in no time!

I also recommend this method if you want to collect a large number of souls in a short amount of time, depending on the amount of enemies, you can collect around 3000 souls in 2 minutes!