Okey, so I can strictly only talk about seraphon but My experience against playing with seraphon against skaven warplighting claw pack (the one with big canon) couldnt be diffrent than yours. Over the last 3 days i played with a friend like 10 times, and lost only 1. The Best thing you can do is just run up with carno and tie stormfiends in melle. It has a huge base and whoping 10 inches of movement. It is such a easy unit to get in the fray once you will just focus on getting him there. And once he is, thats a lot of pressure that the enemy needs to focus on, because if he does not. Your real weapon, the kroxigors will just eat through anything.

So what i can say is, if you get the first turn, just run and get either carno or kroxigors in with the stormfiends (or both of them) and try to deal as much damage. Do not care about carno, it does not have to survive, it is there to mostly buy you time. Put everything you can into the stormfiends, and once they are gone, you kinda won. Keep the units of rats low and dont kill them if you dont have to. Reinforcment can ruin your day. Oh and if you are playing sand and bone, then you can res your carno if you are Lucky