A .22 can be just a lethal as anything else... BUT, it can be very variable in its effectiveness... A couple stories to illustrate...
Worked with (stupid) guy that used to carry a .22 derringer in his pocket...Walked outa work one night, and slipped on the ice...Got home and dropped trou to take a crap, and noticed blood on his under wear...Gun had fired when he fell, bullet went in near his "man parts" and exited near his rectum....He never even realized he'd shot himself.
Had a case here a few years ago, where a guy was on trial for beating another guy to death with a shovel...But, before he beat the guy dead he had been shot 8 times in the chest with a .25 auto.
And finally, in my "young and stupid days", I got into a shouting match in a bar with some idiot...Ended up outside....I tried to walk away, at which point the punk shot me in the back with a .25...I promptly spun on him, and proceeded to kick his ass (last I knew thay said he MIGHT walk again, someday....but his face isn't ever gonna look very good).
In short, while a .22 might be better than nothing, I'd definitely empty the magazine until the guy stopped, if he does. But, if I were you, I'd be saving up for a shotgun...