It would work different for regular NPC demons and Player demons or stronger demons
#Demon execution mini-games
vocal bramble
For a regular npc demon you can easily cut their head off with your sword
but if its a player or a stronger foe, if the demon is near death and you want to cut off its head your blade gets stuck near the demons neck and you have to do a small minigame while the animation is playing (you struggling to cut off the demon's head with your sword)
the minigame could be button pressing or m1 clicking or something harder
This could also work if the demon and the slayer both has to do a minigame and the better wins, if the slayer loses they could gain some speed boost for a few second to try cutting of the head again, or damage reduction (very weak).
full rampart
As much as this is a good suggestion you also should be looking at the negatives of this.
Will this have hyper armor
How can this be abused to avoid being damaged by external threats
Is this appealing to the entire audience
It's a good suggestion but I feel as though the negatives outweigh the positives.