If I am fortunate enough to spot a large predator before it sees me I will freeze and stand very still. I will stay in that position as long as I have sight of the predator.
I would consider myself very lucky to have this situation.
If the animal noticed me and started moving towards me I would get in ready mode with gun in hands. First shot would be in the ground if it continued to come at me. Chances are this would be enough to scare it away.
Once it comes within 50 yards and shows no sign of stopping it's advance then all bets are off.
sailing ... A way to spend lots of money and go real S L O W