Try to save at least three (preferrably four) flare gun shots for this:
1. One in your apartment, then run and hop the balcony
2. Run to the next grouping of TVs and shoot again
3. Run through the hole you've made to the next grouping and shoot again
4. Run through the last hole and take out the boat however you can -- I used a combo of my last flare gun shot, flares, a flashbang, and my flashlight.
I did this and basically ignored the spawning possessed, and breezed by.