The "But You Can't Hide" achievement requires that you kill 100 enemies while they are running away. How can I make enemies flee in order to get the achievement?
The simplest way to get this achievement as a demon hunter is to get into melee range of the ranged demons and enemies you find (this includes Succubi, Skeletal Archers, and others). They'll run to stay away, so you can simply kill them while they run.
No need to get fear on hit equipment. It's an alternative, but getting specific equipment just for an achievement may be a waste of gold.
It's probably better to do this while on Normal (I got it without even trying on act III), since these monsters rarely go without the company of melee hitters, and getting into melee range of those on Nightmare or higher difficulties may be a suicide.
Or you can pair with a Witch Doctor that uses Horrify and have him/her fear while you dps!
Melee weapons, helms, and offhand items* can have the % chance to fear on hit affix. Stack it as high as you can to get this achievement quickly. You can probably find some of these items pretty easily on the auction house.
* Use your browser search function for the word "fear"