Kill the demon Sir on the portal level in a way safer then most... kinda (Important Note: I killed Sir before making this guide, So I can't show it in action but It worked for me. Do this at your own discretion).
Guide to Defeat Sir
Get something pointy and don't bring ANYONE, Unless you want to recreate that one scene in Train to Busan... except all those zombies are one big zombie and has claws and is a demon.
Step 1: Baiting
To start off with killing Sir you need to be swift, I suggest getting manuverability up in the armor tree. You'll need to lead him to this area.
Once you get him there here comes the hard part...
Step 2: Run
Run as fast as you can to this gate. If you havent locked yourself up in a panic shelter than wont open inward... then you probably shouldn't be reading this part while the game is still on... Anyways... get into the door where he can see you, but cant get to you...
Step 3: "Ow... Oh My Poor Toe"
At this point you're safe! Besides the fact you cant go back to level 3 everything is good. Now... Poke.His.Foot... best way to do this is to aim between his two toes, he'll eventually put his foot in the crack on the bottom of the door if you slightly slam the door against his body. Best way to do this is with anything pokey.