This is an undead earth/air aggro deck. You'll want to establish board presence early and overwhelm with skeleton tokens as much as possible (use the genesis on sites that create skeletons and necromancers as well). Once you've made it to your opponent's side of the board, focus as much as you can on their avatar or sites. Stronger opponent minions are going to one-shot pretty much all of your minions.

You also want to use Witching Hour, Necropotence and Gift of the Wolf to buff your minions when attacking the avatar or stronger beasts if possible. Weakening them with Murder of Crows will also help to make them less of a threat to your low-cost tokens and minions.

Use Stygian Archers and Bone Spears if drawn to target your opponent's avatar if possible.

Use Raise Militia mid-game to really get a shit load of skeleton tokens, along with the Master Necromancer who can also create tokens near your opponent if you summon him

Kiss of Death and the Lethal perk of the Plague Rats can mow down larger entities if you're in a tight spot.

UTILIZE YOUR GRAVEYARD! Alot of minions have death rites and Cemetery Rats fills your graveyard early. This can lead to you being able to summon higher power minions for free with Carrionette or other minions.

If you've established board presence and draw Returned King, it's probably over for your opponent, especially if they're also running an undead deck. There's also a combo you can use with Necronomiconcert and Panpipes of Pnom that can really help you and let you dominate mid to late game.

I wanted to get away from the water or air element dominance of the necromancer, as I think the earth minions and spells are more versatile for most boards and opponents. The minions and spells are also very low cost.

Weaknesses will be if opponent's minions are getting warded or if they establish auras (Wall of Brambles) that give damage to your low-power minions at the sites. Also look out for moves that will wipe the board (flooding/submerged).

I've ran this deck against water and fire avatars and even if there were hiccups early on establishing board presence, once your undead machine gets going, it's pretty much a guaranteed win.