To stock up on supplies before entering the Grand Cathedral in Metaphor: ReFantazio, follow these steps:

1. Visit the Water Shop in Grand Trad:
– Buy the three Fruits of Cordial. These items will replenish your MP, which is crucial for dungeon exploration.

2. Stock Up on Medicinal Items and Cures:
– Ensure you have enough medicinal items and cures to manage health during the dungeon.

3. Purchase Elemental Attack Items:
– Visit Gloamhall in Grand Trad, which is only open at night. This is your main source for elemental attack items, which help supplement your MP-consuming spells.

4. Wait for Idlesday for Discounts:
– If you can afford to wait, hold off on purchasing expensive equipment until Idlesday, when shops offer discounts. This can help you save money on essential items.

5. Explore the Nord Mines Early:
– Before reaching Grand Trad, explore the Nord mines. This area is a great place to regain MP without needing to leave the dungeon or change classes. You can also find valuable items here.

6. Complete Side Quests:
– Complete side quests like giving medicine to the sick man in Grand Trad. This will reward you with a hero’s incense, which gives 100 archetype XP and is useful early in the game.

7. Prepare Your Party:
– Ensure your party is well-prepared by leveling up your Royal Virtues such as Wisdom, Courage, and others. This will help you manage resources like MP more effectively.

By following these steps, you’ll be well-stocked with the necessary supplies to tackle the challenges of the Grand Cathedral in Metaphor: ReFantazio.