This mission is tough, but not due to the enemies. You need to keep Bardock alive, as he must survive to beat the mission. This is fairly hard, as you must kill 15 henchmen, then Frieza in order to win the fight.
The only way to get this done easily, is to kill the 15 henchmen as quickly as possible. Items won't work on Bardock, so you need to get through to the final phase before he loses too much health. Once you get to Frieza, don't worry about Bardock and just try to eliminate Frieza as quickly as you can. He isn't very tough, so hopefully you can kill him before Bardock goes down.