More Than A Mouthful - " Fully Upgrade A Weapon " 20g
Some way through Chapter 1 you will be told about upgrading Weapons.
You can then press Start and then at the top of the list will be Weapons.
> Acquired During Chapter 1 <
Vorpal Blade - 175 > 675 > 800
Total = 1650
> Acquired During Chapter 1 <
Pepper Grinder - 475 > 575 > 800
Total = 1850
> Acquired During Chapter 2 <
Hobby Horse - 500 > 625 > 775
Total = 1900
> Acquired During Chapter 3 <
Teapot Cannon - 525 > 600 > 725
Total= 1850
The Vorpal Blade is the quickest way to get the achievement, although personally while i played i hardly used the blade but its up to you to choose which you use :)
Happy Hunting :)