How to survive the first round when starting the game from Round 20. Please make sure that the annotations are turned on.
Steps in video:
1. Run to the Juggernog straight away and buy it.
2. In the same room go and get the weapon MP5.
3. Shoot as many zombies as you can on your way to Quick Revive.(I got lucky by getting the nuke and I took the chance to pack a punch MP5)
4. After getting the quick revive try to create some crawlers by throwing grenade.
5. Once there are only one or two crawlers left, you can try to get another weapon from the mystery box as well as getting some perks.
6. I would also recommend Speed Cola.
7. After this it is up to you to keep going and survive as long as you can!