Soldato
Trying an Empire campaign at the moment and literally cannot beat the secessionist. They can attack me with three armies (I only have one) twice. They'll wipe me out the first fight, then before I even start the next turn they are attacking me again.
I can't recruit enough units to do anything to them without just having to sit there clicking next turn for 15 minutes while they run around raiding my other towns.
How much time you wait to attack the first time? Even on Legendary they are very weak. Build 3 swordsmen first turn and go take out their stack next to your border Turn 2. When you win the battle, build 3 more Swordsmen and attack the town.
Game over. There is 1 more tiny stack moving around but it will try to turtle on their city at West. Just take out the south city and by the time you arrive to their town, you will have a full stack to their half.
Do not be afraid to charge with your Lord. He is a beast.