Since you’re new to the game i suggest that you avoid using shotguns against the Predator, they’re completely useless against them, they’re only good against AI’s.
If you get jumped by a Predator then pull out your knife and try parry the attacks while hoping that your teammates has some kind of brain to come and aid you, this game is about teamwork if you’re on the Fireteam.
Also if you’re in a tiny room then try to body block the Predator so it can’t get out, sooner or later they’ll start realising entering buildings is the worst thing you can do as a Predator.