You're pinned in a sewer junction. Things are crawling out of every tunnel.
Your rifle is wired to the word rack. Build a word — its length picks the tunnel. Two letters fire down tunnel 2, three letters down tunnel 3, and so on round the clock. The last tunnel takes anything that long or longer.
Kills drop gold letters into your rack: double damage. Start a word with the last letter of your previous word to build a chain. No word twice in the same wave.
Every breach raises the sewage and clogs a tile. The higher it gets, the less of the tunnels you can see. At 100% you drown.