Did not manage to kill the mooshroom monstrosity with it in one hit, not even when it summoned mobs oddly enough.
It seemed to go as expected, low damage. Maybe this is all related to the bubble effect since that doesn't seem to work on bosses?
I've tested this again in build 1.15.1.0_9924835 and the instant kill still happens with the specified loadout in the earlier comment.
I then swapped to armor without damage reduction and can confirm it does not oneshot mobs.
So it does indeed seem to be caused by the strange interaction between damage reduction, shadow walker and bubble burster.
Caithlin 
MCD-6330
Wasn't able to see its name at the time, but after looking it up it is the "Vengeful Mariner".
Same happened just now with Thundering Growth and luck of the sea 3. Non-gilded Vine Whip.
Is probably related to the fix for:
Fixed Luck of the Sea enchantment causing Gilded Common items being incorrectly labeled as Unique:
MCD-6000If it's the elevator I think it is in the stronghold, you have to go around and lower the water level via another area.
Not an underwater section like the hidden depths dlc.
Did not manage to kill the mooshroom monstrosity with it in one hit, not even when it summoned mobs oddly enough.
It seemed to go as expected, low damage. Maybe this is all related to the bubble effect since that doesn't seem to work on bosses?
Gear:
Eternal Knife @251
Defeated Mobs Explode lvl2 (gilded)
Swirling lvl3
Artifact Synergy lvl3
Anima Conduit lvl3
Shadow Walker @251
Multi-Roll lvl3
Acrobat lvl3
Electrified lvl3
Bubble Burster @251
Cooldown Shot lvl3
Tempo Theft lvl3
Burst Bowstring lvl3
Artifacts (think only feather was used during testing):
Satchel of Elements @251
Satchel of Elements @251
Light Feather @253
I've tested this again in build 1.15.1.0_9924835 and the instant kill still happens with the specified loadout in the earlier comment.
I then swapped to armor without damage reduction and can confirm it does not oneshot mobs.
So it does indeed seem to be caused by the strange interaction between damage reduction, shadow walker and bubble burster.