[13lackGu4rd]

it greatly depends at the stage of the game you're encountering the skelli spam. early on he won't have the mage numbers for massive skelli spam without destroying his research, so a few thugs with fire brands instead of frost brands should do the trick, the lower enc the better, so undead thugs like Banes are the best. later on when he can afford the numbers you should have adequate research to deal with it. Solar Brilliance is 1 battlefield enchantment you should try to have against undead happy nations, and it also overrides darkness which combines perfectly with skelli spamming tactics. reverse communioned stellar cascades and other anti undead astral spells can work wonders as well. just use the first few turns for soul slays or something than start with the stellar cascades(when the skelli spammers got a mass of undeads spawned already). and of course killing the mages themselves, as others have already said.