So I was trying this strategy I thought of when the worst happened. Time bombs are necessary for this one. I got all my guns and built the tramp near jug. Round 19 with one walker left I grabbed a new tramp and put it down at my spot, then I threw the time bomb. Killed zombies till a max ammo would pop up then blew the time bomb. Grabbed the max and laid down a fresh time bomb, rinse and repeat...
Round 19 with 4500 kills.
Then i hit the time bomb and it sends me to 19 but I'm stuck in the time warp mode where i couldn't move, the screen just kept flexing. Ended up just leaving the game. My point is that maybe there's a limit on how many times you can use the time bomb.