This is obtained at the very end of the game. I believe the achievement doesn't pop until after you beat the game but I could be wrong and it could unlock upon Sadler's demise.
So here's the setup...
You can really tackle this with any weapon combo just short of the knife. Personally, a magnum (Killer 7 or Handcannon), a shotgun or a rifle (sg: Riot Gun or Striker, r: semi-auto), some sort of massive explosive (mine thrower or the rocket launcher), some grenades, and a 9mm pistol of your choosing if you have room. There is a merchant at the base of the tower you climb up to fight Sadler... so get any last minute upgrades in before you go.
The whole goal here is to do damage, drop the steel beam from the crane on him, and do damage to his big nasty eyeballs when they open up. Toward the end of the fight Ada should appear and throw you a special rocket launcher to help finish him off.
After that you'll be treated to a 'smooth' jet ski cruise into the sunset.
Check out this run of it by YouTuber "Mike Bettencourt."