How to keep the Dark Mech gear/equipment set

When you defeat an enemy, you are awarded a fixed amount of experience for each unmastered item you had equipped in the fight, and for each item, that much XP goes toward moving your character to the next level. The total XP is essentially a multiple of your unmastered items. So if an enemy is "worth" 500 XP, you'd get 2,000 total XP if you are using an unmastered weapon, shield, armor, and helm.

If an item is Mastered, you no longer gain experience for using it. So, if one of the items you use in a fight is mastered, then the portion of the XP earned for that item doesn't go toward moving your character to the next level - it is effectively lost.

IMO, the only time you'd want to use a mastered item over an unmastered item is if you are having a hard time with a fight, and the mastered item is stronger than the unmastered item.

To master an item more than twice, you must have the version 1.2 or higher to fight the deathless kings down in the well. When you put the Infinity Blade in, the 3 doors will open and you are able to fight the deathless kings. After you defeat them, you have the privilege to go inside the big door. You fight a robot enemy named Zero Mech. Once he is killed you fight another enemy which is your original ancestor in the beginning. Once you kill him, you can start New Game+. You can re-master any item!