I've gone until at or near round 53 a few times. I find it easier over W@W or BO1 zombies other than the Megatons spawning so erratically at times.
Great way to level up when I've grown frustrated with multiplayer. And the first 10 rounds don't even matter - people could steal all my kills and I don't care - only thing the first 10 rounds are good for are levelling up any weak or least used guns in the game.
Also gets funner and easier as you collect aetherium crystals and max out all your perks, ammo mods, abilities, etc. plus levelling up weapons and getting the attachments that work super well in zombies but maybe not multiplayer.
Also fun just being the field medic. I've had a lot of matches where I've revived 30-40 times.
If you max out jugg, quick revive, and melee with aethuerium crystals you'll be golden. One hit kill the zombies till round 11 while trying to find the mystery box tac-knife, once you do, pack a punch that and keep upgrading it and you'll have one hit knife kills all the way till nearly round 40 I believe.
I can confirm I was phone.