What the achievement asks
Complete an Order of Night delve on tier 8 or higher without being targeted by Artillery fire. Ten points.
This one is personal rather than shared, so a group cannot carry you and your own mistakes are your own.
What triggers the artillery
Artillery fire is called down on the purple circles. Stand in one and it targets you. Stay out of every one of them and the achievement is never in danger.
That sounds simple, and on most of the run it is. The difficulty is entirely in the final fight.
The boss is the whole achievement
The Order of Night bosses drop purple flares that drag the artillery circle towards them, so the circle is not static: it rolls across the ground on a path you have to read. If you are standing anywhere between the circle and the flare it was pulled towards, it passes over you and the run is finished.
That is much harder as a melee character than a ranged one, and harder still as a tank with slow damage, because the flares land in random places and the fight gets chaotic if it drags on.
Kite the boss. Players who got this describe the same approach: keep moving, kill it from a distance where you can see the whole floor, and accept a slower kill in exchange for never being surprised. Skittering Breach's Renilash Beckons story is reported as a comfortable one for exactly this reason.
The story that removes the problem
Nightfall Sanctum, Signal Noise. In this variant you can move the circles yourself, which takes the mechanic out of the boss's hands and puts it in yours.
Kyron's Assault is the other kind one: you are launched up by ballista, and the circles guarding the six shadow barriers never move from where they sit. There is nothing to read and nothing to dodge, only patience while you click each barrier's objects.
Do not port out when the boss dies
The achievement does not always fire on the kill. Several players report it appearing only when they landed on the ground after the parachute at the end of the delve, and at least one hearthed out immediately and thought they had failed a perfect run.
Finish the delve properly. Take the parachute, land, and give it a moment before you conclude anything went wrong.
Where it fits
One of the pieces of Glory of the War Within Delver, which rewards the Ivory Goliathus mount and is part of Worldsoul-Searching, the War Within meta.