Each mission in Minecraft Dungeon sets you and your friends on a linear map where you have to explore the entire region as much as you can to find every secret and available creature that has spawned. The more you explore in the game, the more loot you can receive before finishing it. When you first start a Minecraft Dungeon mission, and you open up your map, you can see how many secrets and chests are available in the level.
The number of available chests can increase based on how much of the location you explore. For example, if you start a mission in the Crypts, you might have access to 12 chests. However, the more you explore, and the more secrets you find, the higher this number can increase as you progress through it. You can choose to follow the standard waypoints in the mission and make it to the end, but you might miss out on some of the chests that could have had excellent loot for you hidden along the sides of a map.
The best bet to ensure you receive the maximum amount of chests during each mission you start, you need to explore the entire map and look for any secrets you may not have uncovered. You can try hovering your character new centerpieces in a room to try interacting with them or look for any greyed out locations in your main map. Unfortunately, you can’t freely scroll around the map, but it does show you any places you have not visited during that mission.
You can repeat missions on any difficulty setting, and doing them at higher levels increases the quality of loot you receive throughout the mission and at the end. You want to try for the highest quality of gear, and to do this, you need to improve your gear power. Your character level does not matter as it only increases the number of enchantment points available to you, and increases your maximum health.