These achievements are require killing 1,000 enemies of any type in any mode with the specified type of gun. There are four main types of guns in the game: pistols, rifles, shotguns, and spread guns. There is one achievement for each. You can see a weapons type before you purchase it in the bottom left of the weapons box in the store. These can be obtained in normal progression, but I myself really only like to use spread guns/Corona Ejector. In this case, they can easily be boosted against the zombies on Bokor in Survival. Use the camping spot mentioned in the "Annihilator" achievement.