I have never shot a space alien but if I knew they were coming for me I would load hardened steel 3/8" dowel pins out of my 375 H&H Magnum. That outta do it.
Just a thought the Minnesota DNR used to suggest that the .280 Remington was a good minimum cartridge for moose.
whelennut

I like to do my hunting BEFORE I pull the trigger!
There is only one kind of dead, but there are many different kinds of wounded.