Cool concept, also I loved the slowdown when bullet gets near me, it gives us the time to dodge at the last moment and that feeling is wonderful.
I have few suggestion
1. In my opinion timer should increase rather than decrease when killing an enemy, as it feels punishing to kill an enemy.
2. Last moment dodge should also add extra time, that would encourage player to take that risk more, creating a gameplay that allows player to take risk while rewarding more.
3. You can also add bonus time if player manages to kill the wave withing first 5 seconds.
All this suggestion will make gameplay more fast paced and action focused.