How can someone deal with mental beat-down?

You should try talking to another teacher about it...(obviously not the teacher that is the one picking on him, because she may be offended if you brought it up and then she'd be picking on you as well) maybe a principal?. i mean you guys are only kids participating in a cl you can't be completely perfect and professional 100% of the time. Maybe next time you notice that she is being a bit hard on him you could talk to this teacher after cl (not in front of other students, teachers don't like being talked against in front of their cl) and say that you think that she is being a bit hard on your friend and that he might do a better job if he could feel less stressed and worried about making a mistake. If talking to a teacher, principal or another adult like a parent doesn't work, you guys should all boycott the cl saying that you will only play for fun, and if its not fun and a bit more relaxed you won't go ot cl... but of course this should only be used as a last resort.