Oh boy, folks. This is a lovely achievement in terms of ratio and value. But it's going to take some work. Fear not, however, as it's not as hard (or extreme) as you might think.
First, I HIGHLY recommend completing a full playthrough on easy before you attempt this achievement. To start, you can get the full experience that way. Assuming you're a completionist, you can also knock out about 49 or the games 58 achievements this way too. On your first playthrough, you should aim to have all factions friendly, and to complete all romances except for Aphra's. To ensure this, I also recommend that you use a female character for your first playthrough. Through some strategic saves, you should also be able to get all but one of the different endings. The one you should have left is A Big Step Toward Peace. This is because you cannot be positively aligned ("friendly") with all factions to earn it, and on your Extreme playthrough you'll focus primarily on story missions (and thus will not achieve friendly status with more than two factions. Finally, you'll get a good sense of what you need to do for each mission, the general play-style, where the good swag is, and what the bosses are like.
Second, your Build
. You should start the game as a technician, as you will be using a pistol and then rifle almost exclusively throughout the game. You can't quite kite in this game, but you can stay your distance and survive much more easily with guns. For your attribute, choose accuracy to start the game. I recommend getting accuracy to level 2 and then alternating thereafter with endurance. These will be the only attributes you use. Accuracy increases your gun damage and ability to destroy armor, which is crucial since all enemies have more armor in Extreme. Your talent should first be science. For most of the game you only need to put one point into this, as you can use the blacksmith in New Serene to add +1 science, +1 lockpick, or +1 vigor to various pieces of chest armor. I recommend rolling with vigor as your main armor (this increases your ability to carry items and ammo, which is essential), and keeping two other pairs of armor with science and lockpick for when you need them. When you can, put one more point into science so it's at level 2 (so with armor you can get this to level 3 and make the best potions, which will help with the last few bosses in the game) and then get vigor to level 2 (which will technically be level 3 because of your main armor set). Then dump a point into charisma (this will already be at level 1 because of the capes that you can buy from merchants fairly early, so this will put it at level 2, which will allow you to finesse many conversations) and then one into intuition. Nothing concerning craftsmanship or magic is useful in this playthrough, so don't worry about those.
For the more visually inclined, here's a sweet video showing your more precise aspects of the build, including where you should put your skill points [not my content]:
The short of it is that you should build up your gun skill [in the technician tree] ASAP until you can use rifles. Then you should build up your stasis skill fully [this is in the magic tree]. Stasis is going to be how you earn this achievement. It completely stops normal enemies until they are attacked, the second point your put into it starts to shred enemy armor, and the fourth point makes it so the next strike is increased by 50% damage. In short, stasis is incredibly useful. Bonus: you do not need a ring or to invest in Divine Magic Ring to use it. With a basic necklace that regens magic you can use stasis about 10 times or so before you need a magic potion (which should only really happen during boss fights). After you get stasis maxed, I actually recommend going for the ultimate skill in the technician tree called Bomb. If you want to get there quickly, go down the Anointed Weapons skill chain. In the clip above, dude recommends traps early on, and the healing skill, but I didn't find either especially useful. In any case, once you have Bomb and the two skills after it, go for Roll between the technician and warrior skill trees. You might want to get this sooner rather than later. It basically makes your dodge more effective. Then spend points wherever you like.
*** You want science at level 1 so you can craft your own ammo. This requires 2 hoof fungus and 1 crude ore per bullet, so gather these things and purchase them as frequently as possible ***
This isn't mentioned in the video guide above, but I recommend that you locate all the skill altars in the game so you can speed up your empowerment. The maps on this site show where the altars are in each location [also not my content]: https://guides4gamers.com/greedfall/Companions
: I rolled with Vasco and Siora the whole game, except for when I completed all of Aphra's sidequests for that romance achievement. Don't bother with Kurt [see spoiler above]. To be honest, your companions will mostly serve as decoys while you hang back and blast away with stasis and your gun. They die all the time, but at least Siora heals you occasionally. This really comes down to personal preference, though I completed all of Vasco's quests in my extreme playthrough too because they are easy, give some nice experience, and give you a bonus to the intimidation talent. I also completed all of Siora's quests (when she's friendly you get a boost to vigor) and Aphra's quests, ONLY romancing Aphra (though you can save and reload to get Aphra and Vasco's romance achievements on this extreme playthrough with a male character if you still need them). Armor
: I won't list all the armor I used here, as you'll naturally acquire better sets as you move through the game. Fun fact: you can change armor with Kurt and even Constantin before you leave Old Serene. Once you get a Sailor Coat, hang onto that bad boy and make it either your +1 science or +1 lockpicking standby. It is also essential to strolling through Naut warehouses with no worries. Otherwise they will attack you and you'll lose faction points. Bummer. Anyway, you should always aim to upgrade your armor ASAP by going to the blacksmith in New Selene. Always go with the purple-tier improvements. I focused more on armor than balance. Now, once you get to about level 14 or so, and your endurance is at level 2, head to Wenshaganaw. This is where I got the Merchant Prince torso, hand, and foot armor that I used for the rest of the game. It's going to require killing a serious mini-boss, but it's well worth it. And with the use of the stasis / gun / repeat formula, you should be able to take him down without too much trouble. Then follow this little guide [also not my content] for where to find that sweet, legendary Merchant Prince set here: https://www.gosunoob.com/guides/greedfall-merchant-prince-ar...
Upgrade these as soon as you can, obviously. For pistols / rifles, these periodically update in the travelling merchant shop you get at the loading screen between zones. I always upgraded the armor damage option over main damage, but that's up to you. FYI: shops upgrade their wares every few levels. I believe once you hit level 25 you can get a 100 quality rifle (Perfect Musket) at the merchant in Hikmet. There are no legendary guns in the game. Wah wah. Lastly, you'll eventually loot a Legendary Officer's Barbut for a helment, though there are some decent purple-tier options too.MISC
Traps: didn't use them [I got the traps achievement my first playthrough, and they're not really effective on Extreme; same goes for phials]
Magic: useless on Extreme. Do not bother.
Potions: health potions are your friend, first and foremost. Apart from these, I didn't use any potions except for boss fights. In boss fights, use stasis and take advantage of the Tactical Pause feature to use resistance potions, magic potions as needed, power potions, and armor ointments as needed. I upgrade my science to level 3 during the latter third or so of the game, and I found transitioned to the greater variant of all of these potions. You might make it through the game without them, but I found them to be vital during boss fights.
*** Do everything you can to become friendly with the Native faction. You should have a good idea of what this entails from your first playthrough, and thankfully you can abuse the save anytime feature to ensure you always make the best decision for them during quests. Just save before you talk to anyone, and reload if you get a any minus points to the Native faction. This should come pretty naturally regardless. ***
Again, the only sidequests I completed were for Vasco, Siora, and Aphra. The rest of the time I focused on plowing through main story quests. This was all enough to get me to level 29 by the end of the game (I hit level 36 on my first playthrough, and that was doing all quests, for reference). This is more than enough to take care of the final boss.
Last thing, just so this is clear: your strategy in every single fight is to lock onto an enemy, use stasis, use your gun, use stasis, use your gun, etc. Always. There is no reason to ever use your sword, except for the very early parts of the game when ammo is in short supply. This will only get you swarmed and killed. Dodge frequently. Thankfully most enemies have pretty predictable attack patterns. Once you have Bomb, use it at the start of every battle. You'll wipe out enemy armor quickly and poison them at the same time, which is super useful. This same strategy applies to boss fights, though you'll need to dodge more and make use of potions. What's really great about Stasis is that you can often nullify a boss attack by using it. This will save your precious bacon many times over.
That's it, folks! Good luck, and good huntin'. You'll probably never use an actual arquebus, so enjoy it in Greedfall. BOOM.