- you could do this on any difficulty (i recommend easy or normal)
- the first possibilty is at the middle of the level where you enter the tank
- drive until you see the weapon crates
- grab the RPG
- now kill 15 people with the RPG
- you always can go back and grab new ammo
- when you destroy a tank will also count towards the trophy (normally you get two kills for that)
- the second possibility is at the hangar at the end of the level
good luck :)