This achievement requires you to complete a number of challenges to unlock minor upgrades. Each perk has five "levels" to it, and you have to complete all five levels to max that perk (for instance, the game says you need to get 10 stealth takedowns at first, but you actually need 50). There are also three perks that you can't work towards until you have the necessary contraptions (these initially show up as question marks).
Here's a full list for those who want it:
*** Spoiler - click to reveal ***50 Stealth Takedowns
50 Thrown Hatchet Kills
10 Takedowns or Tampers (on robotic enemies)
50 Enigma Codes Collected
25 Quick Commander Kills
5 Constrictor Harness Kills
100 Heavy Weapon Kills
250 Dual-wield Kills
50 Combat Takedowns
125 Grenade Kills
100 Overcharged Kills
50 Ram Shackle Kills
50 Kills by Burning
50 Environment Kills
125 Headshot Kills
10 Electrical Stun Kills (requires the emp grenade upgrade)
10 Enemy Grenade Kills (best done by throwing grenades back)
100 Battle Walker Kills
You'll likely get the majority of these through natural play, as long as you keep mixing up your playstyle (especially when you start working on the Killboard), though there are a few you'll likely have to go out of your way to grind out (in my case, I had to grind out grenade kills).
That said, there were a few of these I had to look for a specific place to grind out: Constrictor Harness Kills and Enemy Grenade Kills. Luckily, I found good places to do both of them.
Constrictor Harness Kills can be gained in the Manhattan Bunker District (the same place you get the harness if you didn't choose it as part of the story). Shortly after you get it, go into the vent where the Übercommander is (there will be one other soldier in there with him), and come out where they can't immediately see you. I'd recommend making a save point right here. Fire an unsilenced weapon at the ground, then get back in the vent. The soldier will walk over to where you were and you can easily take him down. Do that five times and the perk is yours.
Enemy Grenade Kills can be gained in the Manhattan Ruins District. Go to the spot in the video below (killing as few enemies as possible), and hide next to the car, and save. There should be 4-5 enemies on the hill nearby. Position yourself so that the enemies can shoot at you, but not actually hit you. This will force their AI to throw a grenade. When you see them about to throw one, move forward and start mashing
to throw it back. If they don't run away, you should get at least one kill from that. It only took me a few reloaded saves to max this one out. Here's the video of me doing this:
Hopefully this helps some people. If you see a mistake or have something to add, let me know and I'll edit (don't just downvote with no comment, that doesn't help anyone).