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After you beat the game, you'll unlock the RPG (surprise surprise its an RPG).
When you go to new game+ you already have it in your inventory, you don't have to go looking for it, all you have to do is get 10 kills. You'll have 20 shots but I'm sure you don't wanna waste a shot on one enemy, unless you're 9 out of 10 on the achievement then by all means shoot away.(overkill but for our achievements we'll do anything)
There are three great places to get this, I got this on Chapter 6 the whole time you're with Joseph, Chapter 12 after you grab the medicine for Joseph (suggested by CRO5573Y) and Chapter 15 right before you face Ruvik (the area with the eye balls and the blood and the RPG/chainsaw)