It is easy to do in 20-22 minutes by round 13. The mods you need are, resourseful, long lasting and MK.II. The more hearts you have the better, but you can do it with 1. Hopefully you have the "Son of Mars" mp-40 but if you don't type 100 will do. Do not leave spawn area untill end of round four. Rounds 1-2 use shovel as fast as you can. Round 3 shoot and/or as fast as you can. Round 4 run around until most of the zombies are out then use freefire, save one zombie. Open gates and get and turn on all valves for gas. Kill last zombie and go to round 5. Kill all pests got to round 6. If you have 2 or more hearts use one now. Run around well until most zombies come out, use freefire quickly to kill and blow well before freefire runs out, save one zombie if you have the son of mars if you don't any smg will do. Rounds 7-13 just run around well and fountain getting the zombies to line up and fill your meter then just freefire. The more hearts the better if you had 4 use the R6, R11, R12, R!3. 3 hearts, R6, R12, R13. 2 hearts R6 R13 and 1 heart R13. The trick is to get as many zombies spawned as possible before you freefire or fill you meter to freefire. I promise this will work. I will play a match with you and let you get the kills and show you how. tag is andysilentmother.
Which is why you should only bother with the 250,400,750 kills and the “survive 10 consecutive waves in the morgue/tower” or “kill a zombie of each type on the final reich” contracts. Those can all be done in a quick,single game and are far more rewarding and easy to do.