Pretty much, any weapon or attack that lacks magical or holy properties would be incapable of harming a Heroic Spirit/ Servant. Physical attacks wouldn't work either since the battalion armies were vampires artificially created through science.
What's the definition of conventional then? For instance, if there was a gun that could shoot lightning or miniature singularities would that still be considered useless if it was created through science? Would that still be conventional? I just find that to be a bit sketchy. Sure, when they're in their spirit forms they can't be effected because of their intangibility and such. But outside of that, when they're in their material forms, why can't they be effected?
Also, even if that was true, the battalion vampires were created using Mina's/Alucard's blood as a template, IIRC. They also perform blatantly supernatural feats, such as walking up and down the sides of walls as if gravity is non-existent. This immunity to conventional means must have a limit somewhere. That's just my opinion though.
Hans and Dandy might be able to hold up for a time before being overwhelmed by Iskander's armies since they're a natural werewolf and vampire respectively. The reload time Rip would have to go through in reloading between shots with her musket would lead her to get slaughtered quick though. The three along with the battalion, just dragging out the inevitable slaughter.
I agree that Iskander would win, but it would make for a better/fairer fight if they were to be added into the mix. It was ultimately a miss match, I guess. Still, I don't think it would be completely one-sided.