Enter The Gungeon

You need to acquire the mailbox gun, which can only be gotten by rescuing Cadance and Ox from a cell somewhere in the Gungeon. They will open the Acquisitions Department which will be on the top floor of the breach to the left of the gungeon. Here you can purchase gear with hegemony credits (dropped from bosses and always in your inventory) that will have a chance to be available on future gungeon runs (similar to Binding of Issac).

Simply purchase the mailbox and pray to RNGsus that you'll find it on a following run. It can be found from chests and even can be sold at the shop.

The mailbox shoots letters, but the last bullet in your clip is a "special package" that will poison your targets most of the time. Sometimes, though, it will shoot glitter. You MUST kill the boss after hitting it with the glitter "special package." If you die, it will void this achievement. I got mine on the Ammoconda boss in the second chamber, but this will probably be easier with a boss from the first chamber.