I read some had questioned whether you saw a lion.I have seen a lion on a hiking trail.
As development and habitat encroachment occur,combined with the fact they are less shot at,the lions are bolder,less reclusive.
We have had one in town near a school,on the edgeof town,a friends kids saw one in the yard.
I find deer on my street and in my yard.If you see deer,there may well be a lion in the area.
I found footage on you tube of a mountain lion trying to take a man.It was trying to go up his back to his neck.Accounts I have read of attacks suggest that is typical.
What I am suggesting:If you see a lion in front of you in time to use a handgun,Its splitting hairs between a 10mm and a 45 ACP,IMO.Having either will work.I used to carry a 5 shot .44 spl snubbie and was comfortable.
What you may want to consider is the one that is on your back,clawing and biting.A big Kabar or equivalent might be more deployable(don't leave the gun behind,of course!)
Not trying to have Tarzan fantasies,just ,like rattlesnakes,the ones you see are not so much the problem,.Its the ones you don't see that are a problem.
Whether this is legit or staged,it gives you an idea
This link will take you to a list of mountain lion/human attacks 2001-2010