I think you would "swarm" them with more detachments. They way I see it is drop 5-8 figures from some of your big units and make them into detachments. Twenty Five should be good enough for most units.
Hey, it is your army play it as you will, but you asked for advice and my advice is if you want to swarm another army, you need more detachments. Those big units are cool. I like your troop variety and applaud you for using Halberdiers.
I think the Seal of Destruction is underutilized. I like it to whack someones BIG spell, leaving them with crap.
Think about it. A level 4 necromancer can toss a lot of dice at a Nehek, or Vanhels. The Seal can eliminate one of these two key spells, making his magic phase less effective.