S
Yes, I'm also inclined to say high diff. I assume mid diff would be too easy given it's an admiral after all; the WG's strongest military force, but at the same time Egghead doesn't seem like the right place for an all out battle that ultimately will result in a lot of collateral damage, on top of Luffy having to recover for a considerable amount of time, in a scenario of a huge marine fleet with vice admirals and Saturn arriving that's not the most likely outcome.
The scenario doesn't seem to be the most appropriate, but I believe Luffy will have to find a solution quickly before we head to Elbaf. By defeating Kizaru in a high-difficulty battle and reducing the numbers of the Marines through the use of Conqueror's Haki combined with Zoro, he will clear the path.