What do you mean? In terms of surviving, Dio far outclasses the zombies in terms of survival. He's been shot, sliced, decapitated, set on fire, and even survived an explosion on a boat as a head. The argument with Boar's bullets shredding Usagi may be a possibility, but it never happened because that was a strategy played in her head, not an alternate timeline, and though the tournament allowed prep time, it didn't allow prior knowledge, and the other warriors only knew a handful of each others abilities through previous interactions, their own abilities, or simply rumors. Boar only recognized Ox, Chicken, Dog, Tiger, Monkey, Sheep, and the Tatsumi Brothers. She had no prior knowledge of Horse, Rat, or Rabbit. Her overconfidence and underestimisation of him got her killed. According to Rat, the only timeline that mentioned killing Usagi was where he and Usagi teamed up and got themselves killed by Horse due to the Rat screwing things up.
When I was referring to Dragon not using his ice thrower, I was talking about when he was alive, and on his own free will. He chose to sit out on the fight and then try to make his move after more than half of the participants were dead. It also doesn't make much sense to let Snake use his flame thrower on Rat or Tiger and not let Dragon use his ice thrower. Though zombies no longer have physical limits or stamina, they are still former shells of humans. I will agree that Usagi's friends may be harder to put down, but in the Jojo universe, zombies are lesser beings and minions to the vampire. Dio can just conjure up a number of his minions to outnumber Boar, Snake, Dragon, and Monkey and keep them distracted enough to take away from Dio and let him fight Usagi one on one.
Without prep time also depends if Usagi had already killed the 4 warriors. If he is allowed to have them, then yes, Dio would have a harder time. The battle would definitely be over in seconds if neither opponent is allowed to use zombies. If that were the case, then hands dow Dio wins due to his intelligence, better arsenal, and range.