Some enemies seem to become invulnerable to damage when the Heavy Armoured mission affix is applied to them.
I've noticed this particularly on Khorne missions with Bloodletters. Heavily Armoured Bloodletters seem immune to damage and on a Purge mission this is an instant fail.
What's stupid is that there are weapons that need a backup, or you will struggle if you try to use them on their own. That's just bad design. All mission have some armored enemies, even if not invincible ones.
I have noticed an ancient power armor enchant that gives a chance of attacks becoming armor piercing, but anyway you shouldn't need that to make weapons functional, and it's based on deflect so not as good for ranged.
but the ability, perk,echant, such thing to grant a armor penetration or break passively,
is what really should be done
Skills with the tags 'Armour breaking' or 'Armour piercing' should do damage to these enemies. There are no other skills if I recall correctly which can damage them. If you run into such a hazard its best to check on your weapons' skills first.