I did a bit of searching today to try and find a mod that would calculate a target's resilience (for arenas) on the fly. The closest I found here were a short post in this thread asking about mob armor/resilience, and this thread here:
Resilience Mod
I'm thinking if it doesn't exist, it may be possible to extend an addon like Mobhealth/Mobinfo to calculate resilience on a target based off of DoT damage, as typically a dot will tick for consistant numbers, and will be reduced by a fixed amount as well. With resisted ticks indicating the number resisted.
If it already exists though (not an armory crawler), I'd imagine it would be of great use in target calling for arenas.