A Flash in the Void

The way I did this in like 40 minutes:
-Delve: Nightfall Sanctum (MUST be Bountiful since all Bountiful Delves have a guaranteed Flickergate inside, otherwise there's only a chance there's one, i think is 1 in 10-15 delves from my experience)
-Story variant: Kyron's Assault is the best since there are Siege Ballistas at the very begin of the delve that are very helpful to reach 4 of the 7 possible Flickergates.
Proceed this way:

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• Make this macro: /run C_PartyInfo.DelveTeleportOut() and put it in a handy action bar.
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• Enter the delve (level doesn't matter until is bountiful) and go in this position: https://imgur.com/gsW9pbF
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• Spot the flickergate in one of the 7 possible positions (for the positions 1 to 3 there's no need of the Siege Ballista, for the other 4 is highly recommended to use the Siege Ballista since speed is the key to do this achievement):
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1. 
   
2. https://imgur.com/WzrVU2K - Right under your position, under the wooden scaffolding
3. 
   
4. https://imgur.com/NDZ0Jex - To the right, close to your position under a big flower
5. 
   
6. https://imgur.com/07FWpWi - To the right, a little bir further near the waterfall
7. 
   
8. https://imgur.com/XSB6eOb - Straight ahead, near the big waterfall (use ballista to go to the boat and drop down)
9. 
   
10. https://imgur.com/zCN01Si - Far left, past the small reward barrack filled with gold (use ballista and drop down to the left)
11. 
    
12. https://imgur.com/1CuzdnS - Straight ahead, just on top of the boat (take the ballista and you are there)
13. 
    
14. https://imgur.com/9ui73m7 - If there's no Flickergate visible in the first 6 positions then take the ballista, take the stairs down and the flickergate will be just in front of you
15. 
    

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• Once you click the Flickergate you can click the macro (no need to fight Ky'veza if she pops out) and you'll be instantly teleported out of the delve, ready to enter in another one in less than a minute, hope it helps
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The best Delve to grind this on is Skittering Breach: Shadow Realm.
Do not click the portal when you enter and no mobs will spawn but the Flickergate will.

EDIT: I should mention this Delve is only the best if you just want to grind the achievement. If you actually want the chest at the end of the Delve there are far better options. For this achievement, it counts when you click the Flickergate, so just do that then Leave Delve and repeat.

To grind this achievement you want to pick a bountiful delve, do it at T1, run past all the mobs to find the flickergate, drop combat then leave delve by clicking on your portrait, repeat.

The mechanics. Flickergates always spawn in bountiful delves. If you complete a bountiful delve it will no longer be bountiful for you, therefore no guaranteed flickergate. Bring a class which can drop combat since you can't leave delve in combat.