Sword and Spear Fantasy is very good and plays a very good game
Shadow Storm, the free fan made addition for Hail Caesar is also very good as, like Sword and Spear the base historical rules are good.
Imuptvs also have a fantasy version that plays well.
Even Too The Strongest has a fantasy version.
All are very good historical rules that can easily be used for Fantasy Mass Battle
Oh and don't forget Kings of War
Good luck