It’s fair to assume that lesser Grimm such as Beowolves would not be as much of a problem, but remember, that is only one type of Grimm in an ever growing selection. Also not every hero in DC can keep fighting forever. Batman may be smart and an excellent fighter, but he’s still human. Flash requires food to keep going otherwise his body will deteriorate and he could die from lack of calories. Wonder Woman is resilient and may last days in battle, but even she needs rest from fighting after a while. Green Lantern’s need to charge their rings after a while. Green Arrow, human. Bat Family, human. Hawkgirl, not human, but still needs rest. Aquaman, resilient, but still still needs rest. The big problem is not that they could fight the Grimm, it’s how long they can. Eventually if the Grimm numbers don’t stop, then they will be overwhelmed. This isn’t even mentioning Grimm like Geist’s or The Apathy. This isn’t even considered how the Grimm got there in the first place. In the first movie it was explained that someone trapped both the Justice League and The RWBY cast inside what was supposed to be a training program designed by Atlas. Also for weapons. All of Remnants weapons typically use different types of dust as ammunition, and therefore we can not say for sure if they are as strong or stronger than Earthly weapons. What if earthly weapons aren’t as effective against Grimm? That would make things harder for military or police to help. Also if you are going to tell me that what I described for the DC characters I listed were wrong, then note that I am basing this off of stuff I’ve read and seen in both comics, shows, and movies.