the final stage of the alien campaign is in fact doable. you can check the wiki page for any tips on it but it requires a lot of luck and patience. i would suggest going for the power ups as much as possible and using the Shotgun which does great damage with Target Marker
this game could be alot better if it had badges more bosses and more and unique weapon updates
honestly finished it in 30 mins and i have to say its not bad but could be a lot better