The special missions in different campaigns in "Call of Duty: Black Ops 6" are different, and Deadly Breed is one of them. If you want to do this mission, you only need to use 12 different weapons to kill in Checkmate. But, it’s not that difficult.
How to do the Deadly Breed mission in the campaign
Deadly Breed
Use 12 different weapons to kill in Checkmate
During the mission you will have a large number of weapons at your disposal, pick up as many as you can and keep killing people with them. You shouldn't have trouble with this. Lethals counting this time, I only used about 5 or 6 weapons and a few lethal weapons.