Complex sentence: General Orellana ordered the musketeers to fire on the men who were directing the launches while the artillery on the flagship's starboard side (from which the fire ship was coming) continuously fire at the sides of the approaching vessel.
Split into simpler sentences: General Orellana ordered the musketeers to fire on the men who were directing the launches. At the same time, he ordered the artillery on the starboard side of his ship (to which the fire ship was coming) to continuously fire at the sides of the approaching vessel.