Players like to go to the Library of Arkthzand for this endeavor, but everyone is so quick to kill every void creature they see that the task takes much, much longer than it needs to.
Here is the fastest way to do it:
When the Shade Colossus appears, DO NOT KILL IT QUICKLY! Instead, just lightly tap it, or block its attacks, or whatever-- but leave it alive.
After a couple of seconds, it will spawn 2 Summoned Shades. KILL THOSE INSTEAD!
After a couple more seconds, it will spawn 2 more Summoned Shades. KILL THOSE!
If you want to go ahead and kill the Shade Colossus after that for the 5th void creature, fine.
But if you want to help out your fellow players, you can just wait and kill 2 more adds-- making 6 void creatures killed-- and leave the Shade Colossus for others.
Fortunately, the respawn interval of the Shade Colossus appears to have been shortened, so if everyone keeps insta-killing the Shade Colossus before it can summon any adds, just keep waiting for it to respawn until you manage to kill it 5 times.
But it is much, much quicker to complete the endeavor by repeatedly killing the adds-- as long as no other players are around to keep insta-killing it.
I've fought mudcrabs more fearsome than me!