I have managed to end the game at turn 28, against 5 AI with a custom sovereign.
My sovereign has
- ugly, stuborn, insane and inefficient weaknesses
- meditative and scholar talents
- 20 in strength, 20 in dexterity, 20 in constitution, 15 in charisma
- basic and restoration spellbooks
- adventurer
- iron broad sword, Pressed iron breastplate, Traveler's cloak and Traveling boots
that allows him to start the game with 40HP, 10 ATK, 8 DEF and 4 moves which is very effective against spider, bandits and others sovereign. Exploring fastly the map allows to find items increasing movement (directly or through agility bonus) or combat bonus.
My charisma has allowed me to recruit all lone recruitable champions.