Remnant II

Short Disclaimer: This is my first solution, and I’m excited to be one of the first to actually get this achievement! I’ll do my best to explain everything clearly. This achievement can be quite challenging.

!Big News everybody!
It seems to be confirmed that it's possible to only join a host, before he crafts the last item to obtain the gun. Meaning you can skip almost all of the work. I mean come on, this achievement is pretty awful. (Reported by Chazzy Motto)
Edit: Just confirmed it. Definitely works, joining someones Ward before he crafts the last item and then obtains the gun.

Location

Spark can be found in the Agronomy Sector/Withered Necropolis on the planet N'erud, accessible through The Dark Horizon DLC.

• My friend and I discovered Spark for the first time on the platform below the room where you assemble the prototype in the Agronomy Sector (below the Repair Lab crystal). You can only reach this area via an airlift. After talking to her once, we encountered her again on the third platform in the large pit (the one with a side route entrance).

• Some people encountered her in various locations; she seems to wander around the map each time you reload the zone, for example, appearing in one of the many alien cornfields. (as reported by Dokkur Firewing)

Spark’s Quest

Background: Spark, identified as 1074-S3, believes she is a failure for not completing her task of cataloging planets.

Dialogue Choices: You can invite Spark to Earth, which she eagerly accepts. However, she mentions a voice preventing her return.

After Defeating the Final Boss:

• Talk to Spark again. It may take a few tries due to her rebooting. Once she agrees to join you, head to Ward 13.

• After defeating the boss, we found her in the room next to where you locate the Nanoplated Set (near the Dilapidated Auditorium crystal), sleeping in a corner.

• Some players have reported finding her there before defeating the boss.

• It’s rumored that turning the purple pillars yellow again might be necessary, but I can’t confirm if this is essential (we did it).

• You may need to reload Ward 13 multiple times after she promises to follow you. In our case, exiting the multiplayer session helped us find her in our individual Wards.

Spark’s Shop

Once in Ward 13, Spark will be next to Dwell’s shop and will offer the Alloy Extractor to collect materials across various planets.

Congratulations on making it this far! That was the "easy" part.

To unlock the achievement, you must craft every item she offers—yes, all of them. This is for her to give you a bonus reward: a stylish pistol. My friend and I jokingly referred to her as the "insatiable *somethingnotnice*" because of this.

Planet-Specific Alloys: On each of the three base game planets, there are three unique alloys you will need:

• Ancient Alloy

• Mythril Alloy

• Alien Alloy

You will need a significant amount of these alloys as the base for the nine secret crafting recipes she offers.
Tip: Farming the alloys isn’t as bad as it seems. Since all players in a co-op session receive the alloys, but only one player has to use them, there’s a handy way to share the load. The co-op players can sell their extra alloys to one of the NPCs in Ward 13, and the player who is crafting all the items can then buy them back for 500 scrap each. With two extra players in your game, you'd only need to farm a third of every alloy, so only 100 in total for all the 9 items. (credit to ICoN xLeViAtHaN for this tip)

When you return from one of the planets, you can share your findings with Spark. She will then ask you to collect three different quest items from each planet for her research, resulting in a total of nine items for nine recipes. She describes these items in riddles, but don’t worry—I’ve got the answers for you!

Item List

Yaesha:

Ornate Lockbox + 50 Ancient Alloy
Fruit of Death + 25 Ancient Alloy
Kolket's Key + 25 Ancient Alloy

Losomn:

Clockwork Pinion + 50 Mythril Alloy
Nightweaver Stone Doll + 25 Mythril Alloy
Crimson King Coin (DLC) + 25 Mythril Alloy

N'erud:

Override Pin + 50 Alien Alloy
House Lythla Glyph + 25 Alien Alloy
Decorum Cipher + 25 Alien Alloy

If you need help locating these basegame (or DLC) items, I recommend checking the Fextralife wiki for Remnant II, though keep in mind that the guides there may not be perfect.

Drop Rates: I’m unsure of the drop rates for the alloys, but we had the most success farming them from large elite enemies.

Final Steps

After crafting all the items (and likely playing through the complete game again), talk to Spark again. She will give you the Redeemer and unlock the achievement!

Not my video but it pretty much explains everything along with item locations.