Description
I made a simple mod that makes you can kill invincible or immortal enemy. Like bugged bandits etc.
If your enemy have -1, 0 or even none health points(empty bar), install this mod and you will able to kill him.

I'm not guaranteeing that will work at all of immortals enemies. I made
this only for bugged bandits fighting with Zdenek in front of Nowhere
Inn. It was annoying. Post a comment if isn't work.

Tip
Try to kill them one by one. I noticed that they do not always die as fighting in a group.
Sometimes you need to spend some time on killing one enemy but in my case I killed three bandits in front of Nowhere
Inn in about 30 seconds.

How to install
Simply paste "modInvincibleEnemyFix" into "...Witcher 3/Mods".

This mod isn't compatibily with others that changing "damageManagerProcessor.ws".
Manual installation:

Spoiler:  

Show

At line 400 change:

else if( actorVictim.IsInvulnerable() )
{
    //don't add any damage
}

to:

else if( actorVictim.IsInvulnerable() )
{
    action.processedDmg.vitalityDamage += dmgInfos[directDmgIndex].dmgVal;
    action.processedDmg.essenceDamage  += dmgInfos[directDmgIndex].dmgVal;
}

next change:

else if( actorVictim.IsImmortal() )
{
    //deal damage but leave victim at 1 hp if it would kill it
    action.processedDmg.vitalityDamage += MinF(dmgInfos[directDmgIndex].dmgVal, actorVictim.GetStat(BCS_Vitality)-1 );
    action.processedDmg.essenceDamage  += MinF(dmgInfos[directDmgIndex].dmgVal, actorVictim.GetStat(BCS_Essence)-1 );
}

to:

else if( actorVictim.IsImmortal() )
{
    action.processedDmg.vitalityDamage += dmgInfos[directDmgIndex].dmgVal;
    action.processedDmg.essenceDamage  += dmgInfos[directDmgIndex].dmgVal;
}

and add below (but before "}"):

else
{
    action.processedDmg.vitalityDamage += dmgInfos[directDmgIndex].dmgVal;
    action.processedDmg.essenceDamage  += dmgInfos[directDmgIndex].dmgVal;
}