Best place to farm these is a level called 'Grave of the Torturer' in Chapter 1. Kill the initial wave of Skeletons. Then two Lich's will spawn (This is a good place to farm 'Lich Slayer') Kill the Skeletons the Lich's spawn and restart the level. There are several explosive barrels here that you can use to kill the Skeletons quicker. You can also unlock the trophy 'Embracing the End' here.
You have to farm all the kills as one hero to unlock this trophy so choose your primary hero. This trophy is a grind and will take a few hours to get so you might want to take several breaks.