There are three firearms in the game (Ebony & Ivory, Revenant and Kablooey). Once they're unlocked, you can switch to any of them by pressing and they are used by pressing . The easiest enemies to kill with firearms are Bathos and Pathos. Use the Demon Lift ability ( + ) on Pathos to take off his shield before shooting him.
Most of the other enemies can also be killed by guns, but I'd suggest to damage them first. Killing them only with firearms will take much more effort. 50 gun kills is very easy if playing on Heaven or Hell difficulty as the enemies die with a single bullet.