Bone cage doesn't take damage from DoT

I was playing a bleed build before and now I'm playing decaying hex/incinerate.
Lady Elswyth is literally unplayable, I just cannot deal with the bone cage.

With the bleed character, my on-hit dmg isn't enough due to support gems decreasing it. I assume the spikes' health are also increased with map modifiers.

With my most recent character, I have no way to damage the spikes.

Breaking pots one by one, that are blocking my way is not optimal, but manageable.
Arastas was frustrating, because I can't damage the barrier in the questline either, but there I can just AA, they don't attack me anyway.
But at Lady Elswyth it's not an option, because I just won't deal enough damage in that short period of time, especially at endgame.

I'm assuming it happens with Asinia as well, since it's the "same" boss.

Honestly I'm not sure if it's a bug or not, but it definitely shouldn't be intended.