You have to get adamantite or titanium armor. Depending on what class your using, try to get stuff like cursed flames, storm bow, or any other good weapons for the destroyer. Now your in hardmode it's also time to start using better healing potions. Also make sure you have a long flat arena high above the ground to fight the destroyer. Use campfires, heart lanterns, maybe even honey. It may be hard trying to farm for the titanium and getting the weapons. Good luck. Also, skip the first 2 hardmode ores and go straight to adamantite and titanium.