Luke gets his ass kicked in ESB because he rushes into a fight without proper training. But yeah, he "learns what he needs" in under six months. That always struck me as kind of lame ... even when I was a kid.

But then, he's not strong enough to fight Palpatine ... he so weak by comparison that I wonder if Ben and Yoda expected that he'd be able to win against the sith, or else they saw him as a lamb to the slaughter who'd bring out the good pity in Vader's heart. You could say that they only intended Luke to fight Vader, but I'd think that they could see more clearly than that ... they'd know that Vader would want Luke to turn more than he'd want Luke to die, and so Luke would be exposed to Palps sooner rather than later.