OK, maybe I completely suck, but...

[Hok]

It was difficult for me too while playing the demo.
The best tactic to win the mission is to exactly do what it asks (let the freighter live 'till the end).
It's important that you understand you just can't win at such a low level if you fight one against three!

Tactic:
Follow the freighter at some distance, keep looking at the radar, then when you see the pirates are goin' for it just intercept them to attract their fire mainly on you. When your shields are low go back in the trail of the freighter's path. The trick is to keep the freighter safe even if it gets a few shots (remember that shields regenerate) while keeping your ship's armor in good shape. This way u let time pass and don't need to leave the freighter alone to go to a spacedock.