The answer is "not". "The company has guaranteed that not one of its employees will lose his/her job due to the merger." This means "absolutely no-one". If you use "no", then you need to change the sentence slightly: "The company has guaranteed that none of its employees will lose..." "The company has guaranteed that no-one will lose..." (Notice how much I've had to change to make "no-one" fit. Keeping "of its employees" is not correct, but now we have an unclear sentence.)

Peachey is spot on.

It is "no one". http://leo.stcloudstate.edu/grammar/nonot.html