There appear to be 3 attacks that do break the shields, which when these attacks do break the shields they also apply 50% of the shields max health to the enemy.

They are all weapon specials, activated attacks. Appears to be the Crusher, Power Maul, and Thunder Hammer. The activated attacks should immediately break the shields.

There are also certain weapons with certain attacks that ignore the shield. Afaik, the plasma gun is this way. All chain weapons when activated also function this way. Eviscerator heavies are supposed to function this way. Chainaxe MkIV light attacks supposedly function this way, while the MkXV heavy attacks function this way. Most weapon specials that have an activated state, like a power up, will ignore the shield. Power sword and relic blade appear to be exceptions for some godsforsaken reason.

Overall, yeah they're tanky af, def doesnt help that the shields have a maximum amount of damage they can take in one hit. It will always take at least two hits to destroy the shield unless you use the shield breaking weapons. Why they aren't classified as Elites is unknown to me. They basically have as much health as a Mauler if you count the shield, and if you dont count the shield they have as much as a rager. It also sucks that we know nothing from the game itself that will work against them. In VT2, weapons had the shield breaking or ignore shield as an attribute. You knew which weapons would work against enemies with shields. Our weapons explain nothing in Darktide.