[Baalz]

Solar rays is an awesome anti-undead spell, basically custom made to fight tougher undead. The answer is something universally applicable - you need to use different units when your opponent is clearly dominating what you're fielding. Instead of leaning heavily on your shadow vestals when your opponent has effectively countered them throw some other stuff in. One route, obviously, is to go with non-undead units. Another is to add in a bunch of undead chaff to take the damage. Solar Rays starts looking a lot less impressive when there's several hundred longdead and pale riders.