Heavy Hitters include
- Zealot of the Lady (the Crow guys)
- Motorized Patriot
There are more than 20 of these in the campaign and most of them are required fights. You should have no trouble getting this achievement in a single playthrough. Make sure to use your vigors, and aim for their head or, for the handyman, heart. Devil's Kiss (fire) is effective against many of these. Also try charming nearby soldiers.