HP Fight may dent Registeel, but one on one, Registeel will still win. And Mismagius is too weak without Shadow Ball - for god's sake, it loses to Rotom, and loses the ability to really hurt Spiritomb, for starters - and loses its most reliable and powerful attack. All this just to beat Milotc and Registeel, who doesn't even lose to that set.
Hell, I just run NP Missy without HP Fight. I've never really missed it, and Thunderbolt works better to deter Milotic and Blastoise, amongst others. Oh, and having more speed than the common HP fighting Mismagius helps too.
Calcs:
+2 252 SpA Timid Mismagius vs. 252/156 Careful Registeel (the one in the analysis) using HP Fighting: 41.8% - 49.5%
+2 252 SpA Timid Mismagius with Life Orb vs. 252/156 Careful Registeel using HP Fighting: 54.4% - 64.3%
100 Attack Registeel vs. 0/0 Mismagius using Iron Head: 44.8% - 52.9%
So HP Fighting does give Mismagius a chance to dent Registeel. If Mismagius NP's on the switch and is holding Leftovers, then Registeel dies first. It'll take two Iron Heads and be reduced to near-death, but Registeel dies first. If Mismagius NP's on the switch and is holding Life Orb, then it takes only one Iron Head and again Registeel dies first. Thunderbolt doesn't deal nearly as much damage to Registeel and is less effective against other Pokemon like Clefable and Steelix, although it does also hit super effective on Moltres.
Now that you argue it Mismagius without Shadow Ball looks pretty ridiculous bar the surprise factor (although you can be sure Rotom switching into Mismagius will be Scarfed, and Spiritomb isn't going to die to Mismagius anyway). It's like a sweeper Alakazam without Psychic. But NP + 3 attacks should still be viable without being walled by Milotic or Registeel.
Otherwise looking at the stats another thing that would break through Milotic and Registeel is powerful physical Fighting attacks. Hitmonlee is an obvious choice here. Absol might work as well, especially if it manages to Swords Dance. Mixed Magmortar with Cross Chop / Thunderbolt would win, too (just throwing out ideas).