I'm a new player. After getting fairly confident playing many games against the AI on the Introductory setting, I started on the Beginner setting yesterday. And I'm not doing well at all. I've been playing a marksman, mage or assassin, and the ranged enemies chew me up very quickly at every point of the game. I can't seem to get anywhere close to a kill before my health is nearly gone and I have to run and recover, regardless of my preparation or surprise attacks. (I don't have much of a problem against melee champions.)
Also, I've noticed that in almost every game there is one player (rarely two) who utterly dominates. This player could be playing anything except a tank or support, and racks up huge kill numbers, much more than anyone else. Even early on, he's wiping out two or three enemy champions at a time, while I'm struggling against one. Some of these guys simultaneously wipe out hordes of minions, well outperforming the players who are expected to be the minion sweepers, sometimes including me. It would be nice to know how this works.