First, I'd gather all weapons and tools that can be turned into one around the house and fortify myself as best I could, then write a Will, containing that upon my demise, finders of my body have it filled with poison and toss it to some vicious beast they can't really handle, so I could take a few with me from beyond the grave.
Once the Will is done, I'd go to some supermarket for food, armed and in numbers, and see if it wasn't already picked clean, for I think people would be fighting each other for supplies just as much as this new unknown threat. As I see in the first few days panic and chaos would ensue making almost all law and order cease, until humanity realized this is a full-scale, Planet-wide war, where every last man matters, so the militaries stared gathering all survivors from all areas. That's what we all had to wait out, or hold out until they gather the people, from then on they'd tell you what to do, they would not leave any other choice either, with us or against us. That's how it could work, to stand united as never before without any Divide and Conquer strategies for past or present grievances or we end up like the Elves in the world of the Witcher.
Then the united militaries of Earth would have a wide range of weaponry that can pack a punch at most land and air creatures and I don't just mean the various guns, tanks and helicoptes, but also rockets sand CBRN weaponry, most of which are banned true, but that doesn't mean the militaries don't have them in some backwater depots. And if they could use them on each other they could surely give a taste to this new threat. The problem would be the areas unpopulated by man like deserts, Syberia, jungles, mountain peaks, underground places and the hardest water and underwater. In these areas we'd have limited options, probably best leaving them be until they get too close to human authority.
With the sentient species reaching peace would be most ideal for they'd no longer be a threat, could help in the fight against the primal ones and could learn a thing or two from them.
Wouldn't rely on magic one bit, because even if some got aptitude for it, they would be untrained and lacking the masters that could help them and we've seen in the books that mages can die to it's excess use even if they possessed centuries of magical training .
Silver, I think would be essential to fight monsters that have the ability to dematerialize themselves like Spectrals and some Vampires. Other monsters could be fought using standard weapons, though might not as effectively.
This war would be long and hard, even until humanity could stabilize it's presence in their formally dominated world, not to mention the difficulty if the gates didn't close.
