Generally it is very easy to win with the Knight no matter what, because you can just cover the promoting square with the Knight and walk the pawn through. Unless the pawn is isolated from protection, then it can become quite tricky and the Rex Solus might come out winning the pawn...
A diagram says more than a thousand words... for example:
1. Nd4 Kb8 [or 1. ... Ka7 2. Nb5+ ~ 3. Kb6 ~ 4. Nc7] 2. Kb6 Ka8 3. Nb5 Kb8 4. a7+ Ka8 5. Nc7#