Dark Alliance

During any mission, open as many (big) chests as you can find and complete the dungeon. After each mission you'll be rewarded with coins and the items you've found. From lower to higher rarity: White, green, blue, purple and orange gear is shown. Once you find an Orange piece, go to your rewards Chest in Base Camp, claim all your items (Hold Y for reveal all your items). Then equip it. Done!

Note: the higher you set the difficulty, the better loot chance you got. Also try to increase your loot chance at resting camps in missions, instead of taking a rest.