Never Enough

To complete this achievement - Never Enough, you must kill 50 Kobyss while having [npc=215254] at your side.

The first time you will be able to do this while you are on the Gutting the Pool quest which is part of [storyline=5527].
The second time will be possible to complete it while the Pool Cleaner WQ is up in Hallowfall.
Quest requires 10 kills and WQ needs 15 to be completed. You can do it slowly, each time WQ is up.
Or you can join or create the raid group so the count for quest and WQ will stop counting, but continue to get credits for the Never Enough achievement.
Once you finish the achievement, you can leave the group and proceed normally to finish the quest/WQ.

Attention!: As for now, In case you will be phased while doing this achievement in the raid group and will not see any NPC or other players, even in other areas of the zone, then you have to go to [npc=215254] in Meraldar's orphanage (you will see that there is no any NPC as well, except the Alyza), go to the second floor and speak with [npc=215254]. Talk to her and use the "You can join me in the Hungering Pool again.". Everything will be back to normal again and you can go back to finish what is left. The way to [npc=215254] you can copy from below.
/way #2215 41.79 55.78 Alyza Bowblaze
Credits go to my friend Ordan, who had this issue, solved and when I had this issue as well, told me what to do.

Complete during the WQ Pool Cleaner to get achievement Never Enough in Hallowfall - The Hungering Pool - /way 50, 50 #Korbyss_WQ

Alyza will meet you there, there is no reason to go to Meledar to "collect her" - [npc=215254] - She appears with a plate helmet on ready to battle.

You can stay in the same area and continue to kill the Korbyss to count towards the achievement, there is no need to be in a raid. I needed to kill 8 more after completing the WQ and got the achievement, and I've been sat on a rock in said area whilst I type this for approx 10mins to so and she's still there with me. Just don't leave the area and you're good.

My main character had done all of the quests in the zone and I got tired of waiting for the world quest so I took matters into my own hands. First, I double checked the orphanage and her house at the pool. She was only at the orphanage in the city and with a dialogue that was not useful (I couldn't get her to go with me). So I got on an alt and went to the orphanage again and she had her quest to go find her teddy bear at the pool. Do that and the following 2 quests and just don't turn them in until you finish the achieve, then it's safe to turn them in. I went into this with 27 and was able to complete the rest pretty quickly.