Ultradon Carnage

You have to wait for the WQ Until it is Done to be active in the western part of Val to progress on this achievement.

There you'll control the [npc=262271] in a vehicle interface with 5 abilities:

[ 1 ] [spell=1287650]
[ 2 ] [spell=1287655]
[ 3 ] [spell=1292609]
[ 4 ] [spell=1287670]
[ 5 ] [spell=1287666]

The achievement is required to unlock the [item=275662] pet purchase for 15x [currency=3316] on vendor [npc=266234] in the Showdown zones' base camps.

As some kills add up to 4% to the WQ's progress bar (which would only require 25 to finish), but you'll need 100 kills for the achievement, you probably want to join a raid group so you don't progress on the WQ while completing the achievement in one attempt.

We get one new WQ every 20 minutes in Showdown zones. They are up for 60 minutes and may return immediately afterward or take several hours. There doesn't seem to be a fixed rotation or a minimal pause timer that keeps them from reappearing.

(Early on PTR, I also progressed on the achievement with standard kills in the zone, without the WQ being active. It's uncertain if this is still a thing when 12.0.7 releases. And I hope that they made changes to somehow phase our characters, because I was already lacking respawn when no other player was around on PTR.)

For those who are wondering, you can join a raid group and not get progress on the WQ but still progress on the achievement. Tested today, joined a random abundance group in LFG.

This can be done on multiple Until It is Done world quests and characters. If you want it done in one go, join a raid group to avoid completing it.