I randomly received the Mailbox gun from a chest. It fires envelopes as normal bullets and ends the clip with a "special delivery", which could be anything from a glitter bomb, an explosive, a poison bomb, or even a fire bomb. Use this on a boss (or one of the bosses, if you encounter the Trigger Twins) and then finish them off to get this trophy. As far as I know, this is the only gun that will give you glitter to satisfy this trophy.