SUNLIGHT! - Obtained the Solar Gun.
To unlock the Solar Gun you need to defeat the FROG Unit and all four of the Beauty and the Beast Unit bosses non-lethally. This can be done using non-lethal weapons (those shown in blue) to empty the enemies' Psyche bar, which is the thin line that appears below their Health bar.
During these fights you also need to collect the dolls that appear after each boss takes on their Beauty form, and in the case of the FROG Unit find the doll in the garage you reach after the battle. Once you complete a playthrough will all five dolls in your possession, the Solar Gun will unlock after the credits have rolled. The Solar Gun harnesses the power of sunlight to fire non-lethal shots and can be charged to blast enemies through the air, dropping all of their items and knocking them out.