A similar method has already been posted but I did it slightly different.
You first have to beat the game on any difficulty.
On the main Menu
Change the difficulty to very easy, or what ever difficulty you beat it as.
Buy the "Infinite ammo" and "One Hit Kills" tweaks.
Then change the difficulty to Crushing, or Hard if you haven't done that one yet.
Then start a new game.
From then on You will always have the cheats enabled and be on the hardest possible difficulty.
I can confirm that this does not disable the trophy, because this is the method I used to beat both Hard and Crushing. (I first played this game years ago and wasn't into trophy hunting that much back then so I just did it on normal.)