Alec Baldwin has won a key legal battle that saw one of his charges over the deadly shooting of Halyna Hutchins dropped — drastically reducing the potential prison time he is facing.

The Santa Fe County District Attorney’s Office confirmed to The Post Monday that it had dropped a firearm enhancement to one of two involuntary manslaughter charges the “30 Rock” star was facing.

Without it, that means he faces a maximum of 18 months in prison for manslaughter — down from the five years he’d faced before.

Lawyers for Baldwin, 64, had e …