User blog comment:Thesupremejoyner98/Grimm that can cut through aura/@comment-3146930-20150113222323/@comment-3146930-20150114231343

When Pyrrha is unlocking Jaune's Aura, we see that the Aura field covers everything on their person, including outlining their hair and clothes. Weapons and armor are also said be conduits for Aura, meaning a person could possibly use Aura to make a shield or sword stronger. A person's hair is also connected directly to a person's head, so it should also be protected.

As far as strength of attacks, it depends on types. As I said above bladed attacks would be much harder to block when compared to more blunt or spread out attacks. The Spider-droid in the Black Trailer used a large spread out attack. Right before impact, Adams tries to shield them with his weapon, but is blasted back out of the train car and Blake takes over while he gathers energy. Also if his Semblance is the ability to redirect energy, then the blast may not have hit them at full power. Ren on the other hand was almost impaled by a very large set of teeth and was forced to focus his Aura into a single point around his hands. Without doing that, those teeth would have most likely gotten past his Aura in a second and even if he wasn't killed, he would be very weak and probably been lunch soon after.

Grimm are very strong after all. In real life, grizzly bears have been known to completely decapitate a moose with one swipe and are very strong overall. Polar bears are even bigger and Ursa in the series appear to be even larger than them. Using this logic with the other Grimm, sure Aura may prevent a fatal strike the first time, but I highly doubt a direct hit from any of the Grimm would leave them with anything less than a few semi-deep cuts.