Fighting Luffy in a death battle is not Kizaru's role in the story. That is Akainu's role. This is precisely why Oda made Kizaru stalemate G5, with neither party doing much, and throwing tons of caveats in for Kizaru specifically. I assume Oda believes Kizaru isn't far from Akainu in PL, so the exposure to an all out Logia Admiral will come with Sakazuki. It's typical of Oda to keep his cards held closely until it’s time for the character he wants to show out.