Regarding undead troops in the Elven army: it's a very good idea to replace Dryads or Dancers with a bunch of Spectral Dragons as higher-tier units.
When I played through it, that's exactly what I did! I replaced the Dryads with Phoenixes (keeping them in reserve so they wouldn't be killed), and I exchanged the Dancers for 25 Spectral Dragons. And everything worked out perfectly.
