Complex sentence: As you pursue him you must fight off gangs of other various street thugs and half way through the level you have an opportunity to save a captive bystander who (if rescued) will help you fight until he is killed.
Split into simpler sentences: As you pursue him you must fight off gangs of other various street thugs. Half way through the level, you have an opportunity to save a captive bystander who (if rescued) will help you fight until he is killed.