Well, here are my thoughts:
Honestly, while I don't think the final episode was anywhere near as terrible as Little East Blue, I feel this promising arc sort of fell apart in the final two episodes. I really enjoyed the first two, despite the obvious deviation from canon material. Shuzo and Alcapachino seemed like they had some promise. I liked Lily as a character as well. Decent crew interactions, a promising side story, etc.
What we got in this final episode was a subpar fight at best. Now, toward the start I had no issue with it. Let me just say that it would be dumb for Shuzo, someone who fought Ain, the second under Z himself, for five days to get instantly KOed by Luffy. I understand some people don't like filler villains being that strong, but since we're in the New World now, it just has to be done. That being said, Shuzo's Haki being stronger than Luffy's really did not bother me at all. Him taking Luffy's Haki punch the way he did made perfect sense and I had no issue with it.
This is where the fight went bad, in my opinion. While Luffy's Haki not being as strong as Shuzo's makes sense, Luffy getting knocked the fuck back by a Haki-coated canon and nearly passing out because of an alpaca is just dumb. People complained about the anime's handling of Hordy Jones' fight with Luffy, but at least that was somewhat justified with him being so powered up on his drugs. Here, I just don't see how a Haki-coated canon ball could take Luffy down so easily. Moreso, how it got LKuffy from the BACK when he's supposed to have Observation Haki and should have been able to easily dodge it. From here on, the fight got pretty bad for me and I'd say it was almost as bad as against the Amigo Pirates, even if the arc as a whole is much better.
Also, while I didn't think I'd have any issues with Luffy becoming a giant, it was drawn weird and felt completely underwhelming. As others said, a simple Red Hawk should have been more than enough to get the job done. While Shuzo taking a regular Armament Haki punch in Gear Second makes sense, if Luffy felt he had to bring out the big guns and used Elephant Gun or Red Hawk, I think Shuzo should have taken a significant hit from that. The way Shuzo was beaten felt very lackluster and easily forgettable, and also made his character look pretty bad.
Finally, the ending was very rushed and I feel like Lily, and especially Pan Fry, were disappointments. It's a shame, because I liked Lily a lot, but her getting beaten so easily before combining with Luffy was annoying. And don't get me started on how weak and boring Pan Fry himself was… A guy with such a huge bounty, regardless of the fact that bounties don't equate to strength, should not have gotten knocked down so damn easily.
Oh well. Z's and Ain's cameos at the end were nice, and I'm still excited to see Film Z. Now onto Punk Hazard and a new opening after the New Year. I'm looking forward to many things.