This is one of the few games where these types of achievements are actually really easy to get. As long as you're mindful of the kinds of weapons that you need to use, you can get the achievements way before the end of the game. And whenever you have access to a heavy weapon, be sure to use it. I also suggest saving the assault rifle achievement for last because you'll probably use those weapons a lot more in the latter portions of the game. But you really don't have to put much thought into it. I often forgot about the achievements, but they all unlocked anyway, so just have fun with it.