To unlock the Professional trophy in Metro Exodus, you will need to kill an enemy with all 10 of the game's ranged weapons.
The 10 ranged weapons available and common locations you can find them are listed below:
Shambler: Semi-automatic shotgun that you start the game with during the intro.
Revolver: Given to you by Ermak on the Moscow level. Can be found often throughout the game.
Kalash: Can be found often throughout the game.
Bastard: Common in the first two missions of the game. Carried by bandits in the Volga level.
Ashot: Single barrel shotgun that can also be upgraded to a double barrel shotgun (both versions will count towards the trophy). Common in the first four levels of the game. Carried by bandits in the Volga level.
Tikhar: Pneumatic weapon that will need to be pumped in order to shoot ball bearings. Given to you by Tokarev on the train.
Gatling: You will use the Gatling towards the end of the Yamantau level when you fight an armoured mook. Burn him with Molotovs until he drops the weapon. Also found on the Caspian level on heavily armoured enemies.
Valve Sniper Rifle: Commonly found on the Caspian level. You can pick one up on the Caspian level on the crane tower when your companion tells you to check out.
Bulldog: Found on the CASPIAN level on one of the bandits attacking the Lighthouse. You'll need to go here while following the story path. And can be found on one of the guards while taking the water tanker in the Oasis.
Crossbow: You will automatically pick this up during the Taiga level.