Complex sentence: His two battlecruisers were the fastest ships present and inexorably began to close on the German cruisers, opening fire at 12:55 that straddled the light cruiser, the rear ship in the German formation.
Split into simpler sentences: His two battlecruisers were the fastest ships present and inexorably began to close on the German cruisers. They opened fire at 12:55 and began to straddle the light cruiser, the rear ship in the German formation.