The Wanted: Krom trophy in Borderlands: Game of the Year Edition is story-related and can't be missed. Krom is the third boss that you will fight in the game. He can be found in Krom's Canyon in the rust commons.
After fighting your way through the valley towards Krom, you'll get the short introductory cutscene and then he'll almost immediately start shooting you with his turret that fires rockets and also has a machine gun.
The best method is to stay in cover, take out any smaller enemies that you come across and slowly work your way forwards. Use a rocket launcher that preferably has a scope on it, and then when you get in range simply keep firing at him.