It's a funny achievement, but someone might not want to go through the game all over again, and with such a melee weapon.

All that remains to be done is to defeat The Lord Of Doors. In the savegame, the magic is fully pumped up, but the Spell Ability is not improved.
Defeating the boss shouldn't be difficult. After you pass, you'll get the achievement.

The link will have a .zip file that is named as an achievement. If you need your save files, back them up. Load the file from the archive into your save files folder and the slot will appear in your game.

Save file: https://drive.google.com/drive/folders/1ubdMO2QPUYo1WCaw38I_f_djO5N9pdAq?usp=share_link

The path for the save folder should be the following:

Academy of Umbrellas
Beat the game only ever using an umbrella as your melee weapon