Live Action Mafia

I bet the necromancers are liches. This could mean that they have phylacteries that hold their life essence that must be destroyed to destroy them.

Alt, behead them, stuff the skull in a safe, and then throw the safe in a lake

It's also possible that you just have to kill them twice (by any method) for them to be fully dead.

Also, I don't think there is any strong reason to believe that both necromancers are liches. One could very easily be a different species.

After being lynched, Adam showed "skeletal features". Maybe we want something that can kill a skeleton? Kill it with fire/burn at the stake sounds like a decent catchall.

So I was thinking that maybe the first death turns a lich into a skeleton (partly due to flavour text and partly based on what happens in Minions, although I would doubt Katie's following Minions that closely). I was thinking guillotine seemed like a good way to kill a skeleton but I don't know.

I feel like guillotine would be a really ineffective way to kill a skeleton. Don't skeleton bones just float around in the air with magic?

But I feel like they have to be connected to each other for things to work properly. At minimum I would expect guillotine to break some of the neck vertebrae, which seems like it would be bad for a skeleton.