A tactic that works well for me is using Rodocks Veteran spearmen and crossbowmen (in your case you may prefer swadian crossbowmen and sharpshooters).
Usually in proportion 40% spearmen 60% crossbowmen, with crossbowmen before spearmen in party list.
The spearmen have huge shields that will take a lot of arrows before breaking. Swadian knight aren't so good at this because their shield is smaller.
Form a line of spearmen and make them stand close to each other then make the archers hold ground just behind them. They will be protected by the spearmen and shoot freely over their heads since they are aiming high.
Don't go too close to the castle / town or it will be difficult for them to hit the enemy.
Have them kill as many enemies as possible (and help them in this with your crossbow). Once everyone is out of ammo make the archers charge. Make the spearmen attack too if you have too many of them in the battle. You need most of them to be killed / knocked down so that new archers with ammo will spawn. Once reinforcements arrive recall the attack and form the two lines again.
If you have surgery high enough you will have far less troops killed than the enemy.
If you have top tier troops it may take some assaults but you will break down their defenses.
Remember to bring quite a lot of different food, meat possibly too.
Regards