Gold cubes can only be destroyed by aligning the lines.

The cube descends every 30 seconds.
For every (about) 30 percent reduction in boss's health bar, the crystal turret increases.
When the chain combo is done, the turret will stop for a short time.
(no enemy bullets erase)

Maybe the most famous combo pattern is 5-5-5-5-1-6 (Numbers mean chain combo counts), but you can build combos relatively freely.
If you aim for "No Shikigami 1CC", the biggest obstacle is also this boss battle.