Welcome, Joelz, and good guess there!
Hector:
Magic is the key. Once you patch, missiles will also work much better. In some patch or other, glamour/mirror image effect was changed to not affect missiles.
Once you can deal damage to the helhirdings, they will lose their mirror image for that battle and your normal troops might be able to handle them. If you have elemental magic, research Evocation (aim for one of: Thunder Strike, Blade Wind, Falling Fires, Falling Frost). If you have Astral, research Thaumaturgy (at least until Mind Burn).