Steps to reproduce:

1. Build up a sturdy Battle Sister easily killing [almost] anyone on +18, in the Enraged build - see my Sister Unia, game nick: unixhorn
2. Choose a +18 tarot mission against either Black Legion or Word Bearers
3. Find out a Relic Decimator there; go to the previous point if none found
4. [March as one, Don't look back, Oden's sons] - Attack!

Expected result:
A Decimator is successfully killed

Actual result:
Can't kill a Decimator, as its shield (the blue bar under the enemy description - is it a shield?) constantly restores, faster than my hero deals damage.

Additionally:

• While a Decimator can't harm me (due to being in Stun and my weapon's 40% Life Drain, 90% Shield Drain), the only workaround is aborting the mission, formally failing it.
• Checked with Power Halberd, Blessed Blade & Rod (less efficient as doesn't have either Armor Breaking or Armor Piercing attacks).
• Also tried to enter Saint Form and use several available attacks, the enemy's shield is decreased, but after leaving Saint Form it restores back to previous values very quickly.
• Used 2-3xPsychon-loaded inoculators with some good effect, but inoculator charges are not infinite.
• Failed to apply a Banner of Vengeance, as it can't be used against Elite creatures (by the way, why it's not documented in the in-game item reference?) even after dropping Decimator's suppression to Overwhelmed.
• Can quickly destroy any other Elite enemy by this hero on +18, including Impalers, Hellbrutes, Fire Prisms, Chonoses, Taloses, Leman Russes, Sentinels, ...
• Can easily kill Relic Decimators on +18 by any other character: Crusader, Assassin, Psyker.

Ideas:
Haven't checked yet with:

1. - Another weapons like Eviscerator, Power Maul, etc.
2. - Berserker build (but expect very low damage as typically those Elite enemies remain alive when their support groups are dead => nowhere to take enough Berkerker tokens)

Environment / experience:

• Game nickname: unixhorn
• PS-4
• The latest game version at the time of reporting
• Non-seasonal game
• Five 100 lvl heroes of 4 classes, total 1000+ game hours: Crusader, Assassin, Psyker, Battle Sister (two of them)

Customer impact:
Customers are very unhappy, especially if such a case happens on a Crusade mission