These crystal-covered enemies can be a real pain but breaking their armor is the key. Bringing strike weapons like hammers or maces makes a huge difference, and remember to two-hand them so your attacks don't bounce right off. When you're fighting them, focus on jump attacks and charged heavies until you hear that satisfying crack of their armor breaking. After that, they're actually pretty vulnerable to regular damage.

For magic users rock sling is your best friend here, since it deals physical damage and can break their stance pretty quickly. If you're dealing with multiple crystalians at once, take out any staff users first, and don't be shy about bringing in spirit ashes to split their attention. Oleg works really well for this since he's aggressive enough to keep them busy.

Something interesting to keep in mind, you can actually use rot or poison to chip away at their health even before breaking their armor. Their biggest weakness is definitely their stance though - a couple good hits with something heavy like a great club will usually do the trick, even if you're not using strike damage. Just stay patient and keep the pressure on until that armor cracks.