Dashing through the Rift This is a lot easier than it sounds, can be done solo or as a group all the same.
To open a Nephalem Rift you need 5 Rift Keystone Fragment's that can be obtained in by completing adventure mode bounties. Adventure mode is unlocked by completing act V in the story. To open the portal you use your 5 Fragments on any Nephalem Obelisk which are located in any town.
Once you open a Nephalem Rift you have to kill enemies until hitting 100%. The basic idea for this is kill as many enemies as fast as you can, not to kill EVERY enemy you see, but do a quick run killing groups. Don't worry 100% doesn't mean all enemies so you won't have to go backwards and search for hidden enemies.
Once you hit 100% the Rift boss will appear and you must kill him. Make a small effort and you will easily obtain this achievement. If you don't get it the first time, its ok because all you have to do is complete more bounties, get more fragments, and you can continually do rift runs.
If you are having a hard time getting this achievement you can always go to the lowest difficulty normal and make the monsters easier. If you are having troubles on normal you can always join a game of people doing rift runs from the menu and they usually do fast rift runs.
Any questions or concerns? Comment or message me good luck and have fun