I try to do the challenges on that site. I must say that I already have learned a lot with them.
Especially the rook + pawn(s) vs. rook endings were useful. I got stuck there for a while.
Thus far, I only knew how to solve from the Lucena position against standard defense (where you bring rook to the fourth rank etc.), but not how to get there from other positions, or if the opponent has also a pawn.
Also, in the challenges, you sometimes meet other ways for black to defend its position, so you have to adapt to get the win.
Finally, I was able to win all 5 of the rook + pawns endings. The knight endings were easy.
Now I'm struggling to get past the bishop endings. I'm still walking in the dark there. I think the clue is to get your bishop on the queening square covered by your king, so that black's bishop either has to retreat or exchange, giving a free way for your pawn to queen. Am I correct?