This should happen naturally as part of the progression of the game, especially if you are also trying to obtain the Marked For Greatness (seal 75 rifts) achievement.
I did not need to farm enemies to do get it.
However, if you want to farm them, my recommendation is to use the Dwarven temples in the Hissing Wastes. There will be puzzles within that require you to light four veilfire torches in a specific order based on the text of the nearby tablets.
if you light the torches out of order, demons will spawn and attack you. As far as I can tell, this will happen infinitely until you manage to complete the puzzle.
This is slightly faster than fighting demons at a Fade rift, and then retreating after the first wave to reset the Rift so that the spawns reset. However,
Note: Users on the BSN report that this achievement does not have to be completed in a single play-through.