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.