Crypt of the NecroDancer

This achievement can be done with any character, so I would highly recommend using Bard since he is the easiest character to play. Mystery Mode is a new addition with the Amplified DLC. It's gimmick is that every enemy and item is replaced with a question mark, so you don't know what you are fighting or picking up. For enemies, they all still follow the same spawning patterns as the regular game modes, so there won't be any surprises there. You can also figure out what you're fighting based on audio cues, like the dragon's roar, movement patterns, like the skeletons moving every other beat, and how much health they have. One note about this is that more than just enemies themselves are hidden, inanimate objects that can be killed, like mushrooms, are also hidden, and in Zone 5, the mages there fire electric orbs that are also replaced with a question mark. That last one can be tricky to figure out, but if you see a question mark spawn in front of another one in Zone 5, that's probably what you are seeing.

Items are a lot harder to figure out, with some being impossible. When you dig, an image of your shovel will show up briefly over the block that was dug. Weapons can be deduced by the range, damage, and whether or not it can be thrown. Some helmets, rings, and boots can be determined by activating what could be their effects or seeing their effects immediately, but not all of them can be figured out. Items like the Ring of War will knock enemies back, while the Ring of Peace will give an extra heart. Armor can be seen on your character itself. Consumables are nearly impossible to tell what they are and you simply have to use them to find out. The only exception is if the item is a stack, like x2 or x6. This indicates it is either a set of spell tomes or cookies.

Overall, this mode isn't too hard, it just requires caution, and as long as you pay attention to what is attacking or what items you have, it'll be relatively easy.