It is practically impossible to fend off an Undead Necromancer when playing as the Swarm. My friend and I have fought several battles and in each one I place a black portal, spawn several clusters of skeletons, then turn them into Wights. I can do this when my friend has just finished going tech 2. I can finish him off before he can tech up.
He can beat me if the game becomes drawn out (if I do not attack him early) and if he uses his synergy to make towers everywhere, but I do not think this should be the only viable strategy as Swarm.
He suggests that the Swarm receive a unit at the start that is not "useless." The Husk is a good meat shield unit, but is absolutely dreadful against Wights.
Also, when playing the Swarm and getting scoprions, when I use the "Fear" spell, it makes my scorpions disappear (as in they are removed from the game) and it tells me that my Synergy spell has been used. It counts as a synergy spell for some odd reason! I cannot use my Swarm RSB after telling a scoprion to use fear. :c