A Flicker in the Dark

Go in the Underkeep on Tier 1 when the story is 'Third Party Operation'.

There is a guaranteed Flickering Gate in the second room that takes under a minute to run to. Once you've clicked it, right click your portrait and Leave Delve to go back outside to the entrance.

Then repeat until you have this achievement.

I was able to get all 100 in under an hour using this method. (I averaged 23 seconds per run playing a warlock with a speed set.)

I’ve put together an in-depth guide for Flickergates here: YouTube Guide.

The fastest delve I've found so far is Nightfall Sanctum - Kyron's Assault Variant.

I was able to get 10 in under 8 minutes. 100 in under 2 hours.

The reason this is so fast and overpowered is because you can use the catapult at the start to launch yourself on the ship and instantly get an overview of the entire delve and quickly see where the Flickergate is, if you don't see it after 30 seconds just leave the delve and try again. You DO NOT have to finish the delve for this, just loot the Flickergate.

Alas looks like this has been "fixed" - the 'Third Party Operation' story is up today, and regardless of whether I go left, straight on or right to the "second room" there was no Flickering Gate. I tried 3 times... fun detected ! AND I just found out that Bountiful Delves "always contain a Flickergate", as detailed on the delve info window when you click [Enter] to enter the delve, noice :) Underkeep is not Bountiful today, so might be the reason why there were no Flickergates /shrug