Tsunami

Each shot throws a wall of arrows and only consumes one from your stack, so the ammunition cost is a fifth of what the volume suggests. The arrows fly parallel rather than fanning out, which matters more than it sounds: at any range, on a target big enough to fill the spread, all of them can connect at once.

This is the single biggest thing to get right. Piercing arrows throw away most of the bow's output, because the first arrow puts the target into invincibility frames and the rest of the volley passes through for nothing. Loading Jester's Arrows or Unholy Arrows can cut your damage to roughly what a single arrow would do.

Non-piercing ammunition is the answer. Holy Arrows are close to the ceiling on a large boss, since every arrow that lands calls down its own set of stars. Ichor Arrows are the other pick: the first hit strips defense and the remaining four cash in on it immediately.