Easiest Way to 500 Proficiency (Also Most Boring) Spoiler Free!
It will take a little time but 0 danger.
Chapter 4.2 three keys mission get all keys and go progress to the next step. Start this quest and you need clamps but instead stay where you are and kill the guardian.
Then attack the plant cluster i use x on playstaion controller its the jump attack with knockdown. Just keep attacking until your 1000 proficiency on all weapons.
This is the most important step by far.
Get coffee lots and lots of coffee to stay awake or even better afk and set up a kb macro you literally press 1 button for several hours.
Good Luck Gamers!