Weapon proficiency is gained in two ways: defeating enemies and collecting items called calibrators. However, you can increase the rate you gain proficiency by purchasing proficiency augments for your suit or finding parasites with bonuses to proficiency gain.
At the beginning of the game, proficiency is capped at level 15 and the cap won't increase to 30 until
, so you won't get this until the very end of the game.
Due to the nature of the game, you may not always find the right bonuses or items, so this may not be possible in every run.
*** Spoiler - click to reveal ***It took me a few runs to the final boss to find the right drops and enough enemies to get this, but after getting an early proficiency augment and clearing all of the rooms in Biomes 4-6, including all three challenge rooms, this unlocked during the mini-boss fight at the bottom of the last big drop before the final boss area.