This battle has multiple sections to it, so be prepared by bringing health items with you, as Frieza will get healed before each section, you will not. The first fight has you fighting against him in his base form. After a short time, he will transform.
The next fight is the same as the first, but now Frieza is in his final form. He is stronger here, but still not at peak power quite yet. Take his health down again to trigger the next phase.
The third section has you fighting alongside Piccolo. With only the two of you, Frieza will be far more aggressive, so be cautious, and play defensively. Lower his health again and the fourth phase will begin.
Now it will be Goku fighting with you. Frieza will act the same as the last phase, so hopefully you still have plenty of health remaining. Get Frieza near dead, and the final phase of the fight will begin.
f you have made it this far, you are likely hurting, so hopefully you brought healing items. Now is the time to use them, as you don't want to die, this far into the fight. During this part, you need to protect Goku while he charges the Spirit Bomb. This can be difficult, as Frieza often goes out of his way to target him. Stay up close and personal with Frieza to ensure you remain his target. Hit him hard and fast, and try to finish the battle before Goku suffers any serious damage. If you find yourself struggling, then make sure to exit for now and level up a few levels. A little extra power can really help here.