Ever done any hiking in high country? Lots of lowlanders get up there and run into serious health problems. Heart attacks are a major item for those not in good physical condition. "There ain't any air in the air, up there."
Get. In. Shape. Or, stay in shape and improve your wind as much as you can.
I started in with centerfire rifles in 1950, taking up loading for a .30-'06. So, I'm sorta biased.
Anyhow, I'd use a 180-grain bullet, sighted in for two inches high at 100 yards. That puts you six inches low at 300 yards.
Lightweight binoculars. I'd use something like 8x30.
My 2ยข,
Art