Okay. I'm not sure how to implement the behavior I'm trying to give the Obliterators.
The intended net effect is "Enemy creature's attack connects with an Obliterator, and this is the trigger that causes GENERIC BAD THING to happen."
I was thinking, let's start with a generic bear, for simplicity's sake. We then add two auxiliary tissue layers that go on the outside of the creature. The outermost of these auxiliary layers is paper-thin, so it should break apart from literally any attack (hopefully allowing the force of the attack to continue through and actually do what it was intended, instead of just being SUPER ARMOR) and it will have healing rate 1, so that it regenerates instantly. The innermost auxiliary layer would be gaseous, and have a syndrome attached that would cause GENERIC BAD THING to happen.
There are problems I can envision, however. First, I don't know if tissue layers regenerate at all while combat is "ongoing". For instance, if I give bones max healing rate, break one bone, and then uselessly grab and release the bear's ear until the end of time, will the bone ever heal? Because if not (like I fear), then this is going to cause bad things to happen. Furthermore, I don't know if gaseous tissues have a different clause for healing than normal tissues (like, maybe the fake-skin repairs itself but the gas doesn't regen until combat is broken). I briefly thought that causing the gas layer to have a second syndrome, one that only affects the Obliterator, and causes it to heal via transformation, but that would have the unintended consequence of healing everything, and rendering it immune to attacks that don't kill it outright.
I also know from experience that [SECRETION] is semi unreliable, and that contaminants can be kind of finicky, not transferring the way I would like, so that workaround is likely unsuitable. Short of forcing every single attack from every single creature to induce a syndrome or interaction that only works on Obliterators and enables them to cause GENERIC BAD THING, I'm not sure what to do about this.