I got around 300+ kills, I did that just by standing at the corner of the map and waited for them. Since they move slow, the turn rate was good enough not to be killed no matter how many spawned.
2 things for improvement:
- Demand more mechanical skill from players: Scale spawn and movement speed higher every 30 seconds, as timer nears 0. Make bullets finite. You could also implement day/night cycle + fog of war.
- Addressing repetitiveness: Shorten to 2-3 minutes. Unless I missed something, give the game an objective other than accuracy and score (find and collect X pieces to win or capture the flag, etc)
Other than that, great job!