5buy them before you progress
The price is tied to how far the world has come, and it steps up twice: once your first mechanical boss dies, and again after Golem. Every attempt at the event burns one, and a failed attempt burns it just the same. So the cheapest way to farm defender medals is to buy a pile while the price is still at the bottom and sit on them. This is one of the rare cases where stockpiling something long before you can use it well is straightforwardly correct.
setting up the arena
Nothing happens unless the crystal goes on its stand and there is enough flat, clear ground around it for the lanes to work. If the event refuses to start, the terrain is the problem and not the crystal. Flatten a wide strip once, leave the stand standing in the middle of it, and every later run is a walk back to the same spot.