After his people were destroyed, a space-faring cybernetic gun-wielding samurai hunts down a powerful old enemy to avenge his people and prevent a devastating alien weapon from destroying all life.

"When his planet is destroyed by space ninjas, the lone survivor forsakes; the path of galactic chi, enters into training to become a cybernetic gun-wielding?samurai to extract revenge on his deadly enemy"

How about ramping up the craziness of the logline: after all, the only way you can have a "space-faring cybernetic gun-wielding samurai" is if that is the hook of?your story,? so you will want to have a plot and a bad guy that matches its weirdness.
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"When his peaceful planet is attacked by space ninjas, a cybernetic gun-wielding samurai must track down the assassins?in order to extract revenge on his ancient?foe."

Great answer by Mike.

I can add: define "his people." How about "his master?" Or "his teacher?" (A samurai can have either.) Or family? Or what?

Probably, the logline's biggest problem is that it is generic. Get more specific with:

1. Who and what the protagonist is (cyber-samurai or whatever, but make us SEE him clearly.
2. What the antagonism / conflict is. Alien what?