This achievement requires you to have 17 "Prestige Points" in order to unlock the final tier of perks. You only need 2 perks for this achievement, those are "Future to the back" and "Modern Warfare". Once you have these unlocked, simply purchase "The Gun v Rock" Building, 1 Riflegnorp and the "Rocket Launcher" Upgrade. Once you have done that, you should see "Modern Warfare" as an arrow type. Select it and wait for your Riflegnorp to fire. This Achievement will then unlock.