The fight to take over Seattle Centre district (District Showdown) will be possible only after D.U.P. forces control less than 30% of a district. Go to the place marked with a phone icon (large billboard on the rooftop) and make a call.
When the battle begins focus ONLY on helicopter which appears in the area. Keep running to avoid missiles which can injure you badly. You can also get down to lower levels (running between buildings), so enemies in the helicopter will have problems to aim at you. As for attacking the helicopter I recommend using Cinder Missile but other distant attacks are also good. Try to destroy the machine before you have to regenerate your smoke, because you could be easily killed during absorbing smoke. When the helicopter is down the battle ends.
Rewards for taking over Seattle Centre to:
- Unlocking fast travel
- Getting rid of all D.U.P. soldiers in the area
- Unlocking Delsin's unique clothes - Fossilized