You should be able to beat 10 without too much difficulty.
You'll want Eldwyin in the deck because her specials are devastating. Make Delth your herald so she turns into Corwin and then have a second corwin in your deck. Furliss is also to have because of her special. Stalker of Worlds, Slaughter Wheel, Void Scrab, Terror Blades, Death Dealers for tear and of course Mountain Wyrms help. Armored Longbowmen and questing knights are good to use as well. You may want to include a rage dragon so it can fly over your units to cover more ground, but you can do ok without doing that. The goal is to have as many strong units as you can manage, kill whatever units you can in the pre-final battles and kill Helena first. Corwin's specials are devastating. Tell everyone to manually target her for the 2-3 turns it will take to kill her. Use Eldwyn's specials and keep healing with her when you can or with Gwydion. Make sure to split up your units to cover ground including Gwydion.