You fight for the Blue Side, and need to capture the enemy fort while holding your own. You control two formations of halbarders behind two formations of musketeers. You also have 80 Conquistadors at the back of the army, and 30 peasants near your fort.
In this battle, enemy forces outnumber you with five formations of halbardiers, and also have access to six cannons. If the cannons get close enough, they can fire grapeshot to instantly kill a large portion of your troops. In addition, if you do not move your peasants into the fort, the AI will sneak it's few dragoons to capture the fort.
To win the mission, have your main forces hold their ground; disable the musketeers attack until the halbardiers get close, and have them fire the volley. Then have your halbardiers approach the front line and make their melee attacks.
Meanwhile, have your two conquistador groups move to the far flank, around the battle. Send them around the battle, and order them to storm the fortress. Your 80 cavalry units are more than enough to remove all opposition within the fort.