Heartbreaker is one of the hardest achievements to unlock in Bioshock Infinite as it requires you to hit a Mechanical Man in the heart and ONLY the heart to kill him. Obviously when you're using crazy crow powers, it gets difficult to be so precise, but luckily the achievement is currently glitched!
Maka has a youtube video here showing how to exploit the glitch in the plaza section of the game. You'll remember this section clearly if you've already beaten the game as there's a bit of back tracking done here. With the sniper rifle conveniently tucked in a back room, you can get the Mechanical Man to chill on a roof top while you take pot-shots at him.