this is really easy and doesnt take long.
first play the game on CASUAL, money is MUCH more frequent and boxes/enemies give your much more.
Ammo is also much more plentiful and enemies die easily so with certain enemies you can simply swing your weapon at them to kill them and with all the ammo you will be getting you can sell most of it at the store and can easily get more. Ammo is worth a lot of credits, for example line rack ammo for 6 of the (which is the most you can carry in a slot) sells for 3,000 credits.
Health packs also sell for a decent amount of credits, smalls for 1,000, mediums sell for about 2,500 credits and larges around even more and playing on casual you wont really need them at all so again sell them at the store
to avoid taking a while simple put a not where you can strategically avoid having to use an extra node on your board, for exaple you can place nodes in a certain way on your weapon and avoid having to use maybe 2-3 empty spaces on the gun.
Lastly if whenever possible buy the vintage suit because you get a 10% discount at the store and it knocks nodes down to 9,000 credits.
Using this method will take maybe 2-3 levels to fully upgrade a weapon depending on how long a level is and how ammo/health and credits you pick up