I'm not all that concerned. The shit humanity faced as cavemen was pretty terrible and we've only gained strength since then. Should we find something else out there, I think our odds are pretty decent. And as super monkeys we have very sharp instincts on tribalism, aggression and adaptability that make us adept at war.

Go ahead, roll out Xenomorphs or Cthulhu. Watch how fast we roll out a blanket of predator drones and thermobaric weapons. Like hell, there's shit we haven't even tried but are pretty confident we could do if we felt like being that violent, we could drop tungsten rods from orbit or send out a nuke with a leaky nuclear reactor that will simultaneously disperse radiation and secondary smaller nukes on the way to its target(no shit thats a real weapons program that got mothballed).

You really think we can't scale up for planetary/cosmic/dimensional violence? In truth, I fear for whatever starts shit with humanity. Most of our constraints are rooted in a shared species and shared planet. If you've got six legs and live on the planet next door, it's not going to be hard at all to get global human consensus to genocide the other guys as needed.