Here’s a really easy mate in 21 that I found the other day on Matplus. Give it a try! The solution is much simpler then the other answer.
White To Move And Mate Black In 21 Moves
[Title "Filip Bondarenko, Feenschach 1960"]
[FEN "8/7Q/2r1p3/2rkr3/2rrr2Q/7K/8/8 w - - 0 1"]
EDIT: @Fate the creator of this answer's question, created an interesting mate in 23 by using double running bishop mechanic to produce a battering ram. Said ram smashes into a Black piece with checkmate
However, the unfortunate thing is that, despite it being a fun problem, the problem, the position is illegal. Thus, I set a out to devise a legal version of the idea that is also, at least, a mate in 20.
Here is the final result of my work. The solution shouldn't be all that hard to find
[Title "White To Move & Mate In 20 Moves"]
[FEN "6bB/3p1pB1/2pP1Ppr/2P3P1/2p5/2P3p1/5pPk/5Kbr w - - 0 1"]
Additionally, just for fun, here is an Ilegal position that extends the idea two more moves than his problem that also uses less pieces. I will admit that his problem is just more elagant looking to me for some reason.
[Title "White To Move & Mate In 25 Moves"]
[FEN "8/2p1p3/2P1Pp2/2p2P2/2P2p2/2pK1P2/B1P1p3/bBpkp3 w KQkq - 0 1"]