You can do this on any level.
Tip: Make sure you have plenty of Health Potions while you are doing this.
The easiest level to do this is Lady Boyle’s Last Party.
The Trophy only popped for me when I was done fighting everyone and not just after killing 5 people.
1. Once you are in the main building, kill the Overseer with the Music Box first. Then run around or just get lucky that multiple new enemies show up to fight alongside the others like me. Just keep killing everyone that challenges to fight you.
You don’t have to just use your sword, but you would be pretty cool.