[Fantomen]

Enc 0 frontline is a good start. Statues, mech men, wights, etc. Buffed with weapons of sharpness and backed by a decent number of priests. Sometimes earth blessed sacreds can do the job. The important part is to have some kind of unit in front that normal undead can't harm easily and that can fight forever.

Archers and flaming arrows are ok but you need to win before ammo runs out and they run forward and gets killed. I think archers are better against pre-raised hordes of undead than against skelespam.

Tough and large enc 0 thugs with charcoal shields are good to soak up undead. If you're using EDM the mechanichal giant is good for this role.

Earth and fire elementals are good against undead too.

Spells like soul slay and paralyze don't target the mindless undead, so if your opponent is using only mages skelespamming that is a way to target the mages directly.

If you can increase the fatigue of the skelespamming mages the wave of undead ends earlier. You might have troops that synergize with Heat from hell, grip of winter, soul drain or even rigor mortis.

Another way is to deal loads of damage very quick before the skelespam reaches critical mass, hoping to kill the mages. Mass flighted lance cavalry or flying troops on attack rear that connects on turn 1 of the battle. Or a swarm of flying thugs with brand weapons.