Hmm I definitely remeber reading in an FAQ a case where dice are rolled one at a time rather than in a pile to work out something similar to this problem, however, I can't seem to find it now and for all I remember it may not even be in one of the current FAQs so that isn't much help.. Surely there is something else in the game with mixed units that this situation applies to. If not, for now, looks like discuss it with your opponent, and since each opinion appears equally valid but serving the player arguing the point, dice off for it until GW rules it.
For the people who are argueing that wounds carry over because they are 1 unit. Does that mean if all your kroxigors die and you are only left with skinks you can regrowth them because you only can't regrow once the unit is dead?
I don't see why not, they are part of the unit. It doesn't make any less sense than bringing back any other model from the dead. This is a magic spell we are referring to. Once again, they are an upgrade, its right there in the army book. This tells me they are part of a single unit. If all your skinks die by the end of the battle, however your Krox are left do you give victoy points to your opponent for wiping out the skinks? I hope you don't because you would be getting cheated. In the case of deploying the slann in a unit of TG and by the end of the game all the TG were dead but the slann survived your opponent would get VP for the TG, but not the slann, as he is separate.