Here are the required epic (blue) materials for each weapon. It is not recommended to waste any on random weapons, until you know which one you'll do the achievement with. Also, it is not worth it to waste any of the rarer green materials on random weapons. You may regret it later.
Assault rifle:2 Pristine Dromaeosauric Patagium
3 Pristine Apex Tooth
Carbine Rifle: (Level 8 unlock)
5 Pristine Apex Tooth
2 Pristine Photophore Gland
Grenade Launcher: (recommended)
2 Pristine Dromaeosauric Patagium
2 Pristine Apex Tooth
Machine Gun (Level 5 unlock)
2 Pristine Dromaeosaurid Patagium
4 Pristine Osteoblast Synthesizer
Minigun3 Pristine Dromaeosaurid Patagium
2 Pristine Apex Tooth
Pistol: (maybe worth a shot, if you get lots of Patagium)
5 Pristine Dromaeosauric Patagium
1 Pristine Apex Tooth
Revolver: (Level 10 unlock)
2 Pristine Dromaeosauric Patagium
5 Pristine Photophore Gland
Rocket Launcher: (Level 15 unlock)
2 Pristine Dromaeosaurid Patagium
5 Pristine Apex Tooth
2 Pristine Osteoblast Synthesizer
Scout Rifle: (if you get a lot of Gland drops, may be worth it.)
5 Pristine Photophore Gland
Shotgun:5 Pristine Apex Tooth
1 Pristine Photophore Gland
SMG:2 Pristine Dromaeosaurid Patagium
3 Pristine Osteoblast Synthesizer
How to farm these materials:1. Try to find a wild t-rex in expeditions. I only found 2 of them in expeditions, but each dropped 2 pristines. The story based t-rex in Vestige-mission only dropped green materials for me.
2. Start an extraction at Vultus Bay with Rosy on normal difficulty, Rosy right now is almost invincible on normal with her RB+LB skill. You just basically don't extract and the dinos keep attacking constantly. Armoured Bull should start charging in at some point. Use this to farm Pristine Osteoblast Synthesizers. Note: if Vultus Bay is at low (green) threat level, it may affect which enemies spawn and attack.
3. Maybe the golden egg in Lata Plains? You can decide if you want to try it, you have to find it once anyway to unlock an achievement, see what the egg drops for you.
4. The alpha raptors drop the Tooth and Osteoblast rarely, but they are rare enemies in the game. So it is not really worth it to try to hunt them. They are those bigger raptors that take more bullets to kill. They do not attack extraction zones.
I found it frustrating to get those Pristine Apex Tooths. I only ever found 3 of them and once the game crashed so I lost one of them. I don't think the story based t-rexes drop any, at least in the preview version of the game.
Some of the more rare resources:Dewclaw:Those red tilted alpha raptors and howlers drop them commonly when killed. These are also found by destroying those blue raptor eggs near rock walls. If you spot a place where these eggs spawn, they may still be there when you reload the game.
Neurotoxic Slime:Armoured bulls are the only ones that drop this.
T-Rex drops these green materials: Tensile Membrane, Solidification Agent,
Enhanced Sinew, Potent Pheromones, Solidification Agent. It may be worth it to grind the Vestige-mission T-Rex to get these.
Jackknife Chelicerae:Those alpha raptors can drop these. Also something else, can't remember right now.
Jawbone:One of the dino types drops these sometimes. They can also be found in those blue raptor eggs.
Mutator Agent:Either the regular bull or armored bull drops these.
Reinforced Plate:Regular bull drops these.
Map: