Not sure I can do aura, but I'll go re-test stoneskin, shield and resist element, and log the whole thing. Unless the staff has tinkered with the numbers since a few years ago when I tested this stuff, what I posted is accurate (*).

(*) Since the amount of damage I was using to test was pretty small, it was not possible to get "exact" percentages, depending on how they do the rounding. All I could do is narrow it to a certain range.

Suppose I knew way to do exactly 20hp damage to myself. I eat a prep, and now I only take 15hp damage. That looks like 25% dam redux. However, if the calculate damage by truncating the decimal portion, then it could actually have been 15.99 hp. Or if they use the ceiling, then it could have been 14.01 hp. So the actual range is "between 20% and 30%".

In the case of my actual testing, the ranges I came up with were:

aura/shield: 16.67% to 23.33%
resist element: 30% to 36.67%
stoneskin/bardresist: 20% to 26.67%

What I can definitively say (again, unless they've tinkered with it since I did my testing) is that resist element definitely gives more damage reduction than stoneskin/bardresist, and that stoneskin/bardresist definitely gives more damage reduction than aura or shield.