A popular strategy is to run around in circles in a large area until all of the zombies are behind you, then shoot what ever bullets you have into the horde. When you run out of ammo, switch to a thundergun, or a ray gun to clear the horde.
The bonus mode called Nightmares in Call of Duty: Black Ops 3 was a reskinned version of the BO3 campaign with a separate story, but it didn't gain much popularity due to a lack of unique content and original characters. Lessons should be learned from Nightmares for future attempts at a Zombies campaign.