Best place to farm Demons is a level in Chapter 3 called 'The Demon Pen. You can either clear out the first two waves and restart the level or there is a method for farming these while AFK. See the video. It is recommended to use the Wizard for the AFK method, you can also farm out the Wizard related trophies here as well.
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.