I'm not sure if it is harder, but I think there are more creative ways to solve this one than either of the first two. With a weird approach, I was able to get this one down to one line. The others were almost trivial to get a single-line solution.

I think it's harder than the others, the pawn and bishop one took me one line

Not really. Chess Katas get easier the more you do them. This is quite easy.