On this page of our trophy guide to Ghost of Tsushima you can find a detailed description of The Perfect Storm trophy. This trophy is a reward for improving the main character's sword / katana in 100%. Below you can find information on how to meet these requirements.
Jin will use the same sword throughout the whole game. It's the Sakai Katana, which you acquire at the beginning of the story campaign.
Important note - As you make progress through the game, you will find or receive Sword Kits. These are not entirely new katana swords. Each set found allows you to change the appearance of the katana and the tanto dagger. Weapons statistics won't be reset and you don't have to improve the katana from the beginning.
In order to improve your katana, you must visit the Swordsmiths. They can be found in many places on the map, especially in larger temples and villages inhabited by many NPCs. Each Swordsmith offers same services
To make each enhancement you need supplies and crafting materials. Unfortunately, these requirements will increase with each level and to unlock the trophy you need to upgrade your katana to level 7.
The most problematic component required for crafting is gold. It is a very rare material. The best way to obtain gold is to recapture locations controlled by the Mongols. After hovering the cursor over the location on the world map you can find out more about the rewards for taking it over. Look for those locations where gold is one of the main rewards.