It's probably a little late since the Halloween event is ending, but my friends and I spent the past couple days figuring out the scoring on the event.
There are 3 main sources of points on the brawl:
- Door Health
- Taking damage on the door results in a static loss of points, as well as losing the potential points for killing the zomnic/shock tire that damages it.
- Survival Bonus
- There are incremental survival bonuses that are awarded throughout the game, up until the last boss fight.
- These bonuses increase over time as long as nobody dies. When someone dies, the bonus is reduced significantly.
- Elite Bonus
- The speed at which you kill each boss adds a bonus on to the total score.
- There is a max of 100 bonus for each Reaper, 250 for Roadhog and Junkenstein each, and ~200 for Mercy (maybe more?)
Additionally, the Elite and Survival bonuses seem to get multiplied by some factor when the final score is applied on a win.
I don't know exact calculations of the various final bonuses, but I can say that to get a high score you should:
- Die as little as possible
- Kill elites as quickly as possible
- Don't let the door take damage
A 5-star score is possible with deaths, door damage, and/or slow boss kills, as long as you make up for it in good performance in other areas.
Some reference playthrough videos: