There is and issue with how Barkskin works when a creature or player in low on health, so as a barkskin user gets close to death the effect of barkskin increases massively.
Here is what I mean If I have an infliction spell on a target and after the effect of bark skin has reduced the damage it causes 30 damage (100 - 70%) when the creature is at full health all is fine, next round damage will be 30 again.
BUT if the target gets to a point were its has only 20 life left then you see something odd. You would expect the next time the infliction hits that would cause 30 damage now does a lot less. I would appear that Barksins 70% reduction on inflictions is worked out last after all other damage buffs, but also with the way the game deals with final blow damage.
Final blow damage is that if your attack does more then the remaining HP of the target then you only do damage = to remaining HP
So if you can damage a target for 200 damage with Solar spear and they only have 320 HP then the first SS hits for 200 damage but the next can only hit for 150 even tho it can do 200.
So back to barkskin issue.. If your infliction would normally do 30 after the 70% reduction once the target gets to 20 is seams that the game changes the amount of damage from the original 30 now down to 6 (70% off the 20 remaining HP and not 70% off the initial 100).
Only solution to this I can see is to change how Final blow damage is worked out, so if the spell does 120 damage it show that the attack did 120 and in effect could have taken the target to -50 their still dead wither way.
same problem affects mirrored destiny