There are a huge number of ways to spend your diamonds and free coins in Blade Idle, but here MuMu Player will tell you the most efficient way to progress through the game the fastest way possible.
Summons
Costumes
More Summons
Only focus on summoning Legendary (Purple) or Mythical (Gold) accessories for the time being. This is a great chance to get ACU and PEN for clearing stages.
Spend your free coins on costume packages in the shop tab, they cost 230,000 free coins each. There are currently 5 costumes that will cost 1,150,000 free coins.
Purchase free coin costumes in this order:
I recommend purchasing any excess free coins on weekly growth packages that contain Ancient Insignia Fragments for 60,000 free coins. You can purchase this twice every week.