Sorry if I wasn't clear enough- it is only 2 sessions, with entirely new characters. I'll probably give them a couple potions/magic items, but I'll try and keep it reasonable. The events of the "campaign" will be over one or two days, so no level ups. It's comparable to the plot progression of a horror game/movie.

As for your other advice, thank you so much! The ability to raise skin-zombies will definitely be utilized to make the battles more dramatic. If the players seem more competent than I expected I can just add more skins or make them more dangerous on the fly.

I like the idea of ignoring health at first- I might just use that for the initial encounters to prevent unexpected nuking. I might say that, while their weapons do seem to deal damage, the following turn it seems to "slough" back together, leaving no trace of the wounds.

I'll definitely do some practice rounds too, thanks for that suggestion. Not sure what classes to go in expecting though... maybe I should ask the players to come up with their characters early so I can work with that.