What you need to realise with this game is that repeated loading of saves doesn't reset the kill amount of any weapons.
The first Turrets you'll come to are early on located in the Barracks. There is an area where you'll find a prisoner to release from a cell (which will give you the ' cold comfort' trophy). In this area, in a corner, is a turret.
If you leave the prisoner alone for the time being, save your game, then man the turret, you'll be attacked by around 6 guards. Kill them, load save, rinse and repeat till you get to 50 kills. Should take no more than 5 minutes.