475 kills with no per-weapon counter: the rule is to keep every new weapon until 25
Eleven weapon trophies: which biome each class appears in, which trait or alt fire solves each challenge, and why Full Arsenal and Soltari R&D demand picking up and firing everything you find.
And it counts towards Soltari R&D, Enforcer, Relentless, Spread The Cost, Vicious Circle, All Shall Bow, Broadside, Atrophy, Angle Of Attack, Radiance, required for the platinum.
Full Arsenal and R&D: the pick-up-and-fire rule
Full Arsenal (11.1%) asks for every Main and Power Weapon and 25 kills with each: 15 Main and 4 Power Weapons, 475 kills. The trophy has a tracker on the list, but the game does not show kills per weapon. Hence the rule: every new weapon you find, keep it until 25. If unsure which is missing, use one and check whether the trophy progress rises. The weapon list is under Tutorials.
Soltari R&D (36.6%) asks for every trait and variant of a single weapon. Each class has 3 variants — Handcannons are Eruptor, Ricochet and Marksman — and 12 possible traits. For it to count you only need to pick up the weapon and fire it: no kills required, but you must actually shoot.
The two rules merge into one habit: pick up and fire every weapon you see, even if you will not keep it, and hold onto new ones until 25 kills. With that, both trophies fall without planning.
The game does not show kills per weapon. Keep every new weapon until 25 kills; if unsure which is missing, use one and check whether the trophy counter rises.
The nine challenges, weapon by weapon
Handcannons, Enforcer (65.4%): damage 7 enemies in 5 seconds. Available from the start; a pack of Vessels, the floating tentacled enemies, does it.
Rifles, Relentless (79.5%): 50 consecutive hits. They autohit: you only have to stop shooting once the enemy is dead.
Shotgun, Spread The Cost (12.1%): 7 consecutive blasts with every pellet landing. From the Ancient Depths. Right up against a large enemy, like the Osteomaw. So few have it because almost nobody fires point-blank.
Crossbow, All Shall Bow (49.5%): 3 enemies in one shot. From the Blighted Marsh; the Repeater Crossbow's alt fire penetrates lined-up enemies.
Chakrams, Vicious Circle (36.5%): 4 enemies in 2 seconds. From Acolyte's Haven; the Split on Hit trait splits the disc into several.
Prominence, Broadside (51.8%): 5 kills in one blast. First Power Weapon, Shattered Rise: into the middle of a pack of Vessels.
Dispiritor, Atrophy (57.2%): 5 enemies at once. Second Power Weapon, Ancient Depths: releasing the trigger leaves vortexes where you shot. Spray the group and let go.
Nova Lance, Angle Of Attack (43.0%): one kill by ricochet. Third Power Weapon, Desecrated Fortress: aim at the wall beside an enemy.
Illumine, Radiance (33.8%): 3 enemies at once with the beam. Fourth Power Weapon, Cathedral; the Chaining Arc trait makes the beam jump between enemies.
The trophies involved
Each one links to its section inside the full Saros trophy guide.