Great work. Very atmospheric indeed. I especially enjoyed the vocals introduced near the last minute. The light off-tempo percussion in the beginning was an interesting touch, and so was the speed up.
The problem I see, though, is that I can't imagine this as a battle theme at all (especially for a bullet hell shooter), as I feel it lacks the necessary tension/intensity. It could work wonderfully as an ending theme, especially if it were longer. I would love to hear an extended version that builds up to something even more intense.