Combine the tactics of the 3 above posts.
Faerie Dragons prevent necs from casting skeletons, unless they want to do heavy damage to themselves and nearby mana pool units. Will also deal damage to the death knight if he tries to heal something.
Dark Ranger rocks vs this strat since Silence hurts. You can also use heavy hitter spells like Roots or Shadow Strike to hurt one nec severly, maybe even kill it. Less necs = less skeletons summoned at once.
If you have a warden, try to fan of knives the necs. Doing 75-190 across the necs is devastating. Hell, one time me and a guy killed an entire squad of necs doing FoK and SSing thier dreadlord, killed them easily.
If you still are facing skeletons, manual Abolish on them.
Also, as N said - Level 6 demon hunter works wonders here. His attacks splash and he regains health much more effectively in his demon state. Also he has more Hit Points!! A truly devastating attack! Also, try to not let your units because thier skeletons! Healing and micro is important.
Also, use wisps to not only kill skeletons, but to destroy the mana the necs have. 50 Mana is quite a lot, even if they master trained.
Or you can try to rush the guy so he cannot do necrowagon.
For thier heroes, watch for coil/frost nuke or the dreadlord with his Sleep and Vamp Aura. The vamp aura will make the melee skeletons a lot more annoying to kill.
In other words, dispel the skeletons and go for the real threat. Also don't let MW blow your men to guts.