Lots of kills. I even sent off to Acornsoft and received a badge through the post. Or, there was the hack where you could just rewrite the save file which was

Lots of kills. I even sent off to Acornsoft and received a badge through the post. Or, there was the hack where you could just rewrite the save file which was published in The Micro User. You could even tweak the missiles, power of the laser and a few other bits IIRC.