I want to get as much Demonite ore as possible to make Shadow armor and to sell the leftover ore, but there is no way I could get enough from mining.What should I do?
Definitely the best way is defeating the eater of worlds. He drops a ton of shadow scales and demonite ore. He even drops some when you've done some but not all damage to him. You can also rarely get armor similar to demonite armor from corruption enemies. If you didn't know how to spawn him, here's a link to the boss: