If you are failing the mission because Bardock is dying you should do Hercule mentor to get his ultimate skill, called "The Savior Has Come". If you use this ultimate all Frieza's soldiers will lock-on on you and stop atacking Bardock, then he won't die and you won't fail the mission. You should use the skill more than 1 time because when you kill 1 soldier other one will spawn and he didn't get the skill effect so he will atack Bardock.