There are three approaches to this challenge:
- Wait for the next phase of the invasion to start, which will no doubt increase the spawn rate to the point where you can't AVOID completing the feat of strength.
- Fly around looking for a rift that is open. Each zone tends to host rifts of a specific element, so you can look up which zones you need and just go roaming. Northrend is the most crowded, so open rifts are likely to be found and killed before you can reach them. The old world is the least crowded, but you can't use your flying mounts so it will take longer to search. Outland theoretically sits in the middle, though I didn't have much luck when I went there.
- Find a very well known spawn point, such as the gnoll camp outside K3. Park your character there and alt tab out to work on your blog. If you're lucky, there'll be dozens of players hanging out in the same place, slaughtering anything that would otherwise pose a threat to your AFK character, and then slaughtering the rift itself in seconds when it finally spawns.
The scene while waiting for the rift to spawn, many more players actually showed up before it finally appeared.