This was always gonna happen.
Firstly, Oda respects Garp too much to have him outright lose his first on-screen battle vs a top-tier. So he needs to cheapshot Garp to allow him to save face.
Secondly, for Kuzan to defeat Garp (swiftly) he'd most likely need to reveal Logia awakening and Oda's not willing to reveal that yet. So instead, he has Kuzan fight Garp in a clash of haki which is actually a ridiculous feat for Kuzan if you think about it.
Shiryu has absolutely no place in this battle between mentor and student. So his role will always be to nerf Garp enough such that Kuzan can take the W without going all-out, that way everyone gets to save face and we get a swift conclusion.