Hitsugaya has always been considered one of the lower tier captains, but he is still a captain. Giselle confirms that he was indeed turned into a zombie before he died, meaning that he retains much of his original powers and ability, minus his consciousness and personality. This means that much of his usual immaturity is gone: black Hitsugaya strikes very decisively and ruthlessly, cutting down both Ikkaku and Yumichika almost instantly. Even Charlotte severely underestimates his “cute” opponent and is gravely injured before he can react. Without the bullshit chit chat before the battle and in-depth analysis of his own abilities, he turns into a very effective fighter, more so than he has ever been before; it’s a shame he’s on the wrong side of the fence. Even though Ikkaku and Yumichika are only lieutenants, keep in mind that he was completely knocked out after his previous fight and has very little reiatsu remaining, so his strength is somewhat surprising.
Of course, Kurotsuchi doesn’t care much about the well-being of his fellow captains, and as it turns out he’s even more sadistic than Giselle, telling her that there’s no fun in controlling someone who’s unconscious. He’s pissed at the loss of his zombies, and the Gotei 13 is lucky his actions are usually at least somewhat in line with their interests. It just wouldn’t be Kurotsuchi if he wasn’t planning to do horrible things to Giselle and her shinigami zombies, all “for the sake of Sereitei”. The other two arrancar are still around and they may or may not fare better, but in any case, I’m confident that Mayuri will ultimately win this fight. Unfortunately even if Hitsugaya survives the mad scientist, he’s going to be in really bad shape (and suffering from a good deal of guilt and shame for turning against his allies).