Mongoose will kill cobras more often then cobras will kill mongoose. The mongoose are born snake killers and are part of their prey species. They have extremely fast reflexes and are very intelligent, and know how to use the snakes weaknesses against it. They keep their eyes completely on the snake and watch its movements and can almost calculate exactly when the snake will strike. Also mustelids like the mongoose seem to have some kind of resistance to snake venom. So even if the snake managed to strike the mongoose say in the leg I doubt it would kill it....