Just space your characters a tiny bit so they don't eat splash damage and play the mechanic.
When you play the mechanic he will become vulnerable to physical attacks and die very quickly.
The mechanic requires you to hit with two different sources of damage types to make the mixture 'rotten' after attacking with the rotton mixture Thisobald gets more inebriated. Do it 3 times and Thisobald will become incapacitated and take double damage including from physical.
