"Sink the Spanish ships" is the last mission objective in Sequence 3 / Memory 06: Proper Defenses
Walkthrough[edit]
You need to sink 8 enemy ships
Use all your firepower against the eight gunboats
Try to keep moving, circling the enemy ships and constantly firing. Since the enemy ships are grouped together, you are likely to hit at least one enemy ship each time your cannons fire. And since you are on the move, the enemy ships will have a hard time hitting you.