BioShock 2

The Preacher is the boss of Siren Alley, found in the "Temple of the Lamb". He's more or less a Spider Splicer with a ton of health and is accompanied by a security bot. In his room are also a Brutish Splicer (can be pulled without the boss) and some other trash mobs. A healing station is to the right inside the room that can be hacked quickly after dispatching the Brutish Splicer (or before with autohacking). Don't save on medikits/eve syringes, there are loads laying around after dealing with the boss. Be sure to use your camera on the harder enemies if needed, the encounter gives quite a lot of research points.

Story related achievement. You will get this in the church level after killing the preacher (Boss). Once you kill him and his followers it should pop.