I'm fairly certain PHF will get adapted as an OVA of some sorts eventually, and nothing Araki has written has contradicted it. Also, he was somewhat involved in making it.Actually, almost right after posting that I realized that just about every member of the gang except Abbacchio and maybe Narancia would have ways to deal with Metallica.
-Giorno could transform random objects into fresh blood to replace the iron torn out of him, or even nullify Metallica's attacks outright by turning the blades Risotto forms right back into living blood before they can damage him.
-Bruno could just unzip himself to pull the blades out without needing to damage his body (plus the zombie thing you mentioned).
-Mista would have harder time, but the Beach Boy/Grateful Dead fight shows that Sex Pistols can go inside his body and move around, so he could use them to pull out the blades, though not without taking some damage in the process.
-Narancia wouldn't have any direct counter to Metallica, but the Clash fight shows he can keep his cool and attack even when his neck is being savaged by a miniature shark. It'd take more than vomiting up a few razorblades to get him to let up. Plus Aerosmith's radar would let him track Risotto even while he's invisible, removing one of the latter's major advantages.
If anything the whole deal with Risotto shows one of the things I like most about Araki as a Shonen writer: he's not slavishly bound to "power-levels" as a means of deciding who beats who in a fight. Most of the main cast would've been able to defeat Risotto fairly easily in a straight fight, but he still manages to get within a hair's breadth of killing the main villain, purely through clever tactics and the fact that his ability was almost perfectly geared for the setup of that particular fight. It shows that in a real fight, things like tactics, preparation and how well the respective opponents' skillsets counter each other can matter just as much or more than who's technically "stronger" on paper.
Also I was aware of the PHF spoiler, but A: Neither Bruno's gang or Risotto know that, and B: it's a spinoff not written by Araki himself and thus doubtfully canonical.