@wishie I had the same problem. I only managed to beat the level with golden 30s. Be sure they can instakill the ogres. Wipe the ones on the left first, kill the one on the top and then go to the right. Be sure to kill all the ogres that are around. Stand beside the banner and kill the monsters the second they spawn. If there are no monsters on screen when the clock reaches 0, you win.
After countless attempts I finished Alien mission 15.
I used katana to deflect razes' rockets right after his shield goes down and for some nice melee kills. Then I equipped Holy grail (good for getting rid of the adrenaline boosted human really fast) and of course rocket launcher. I found use for the assault bouncer and the plasma shot but only to finish off low health foes quickly.
The power extender was very useful. I tried to catch as many power ups as I could, trying not to waste much time waiting for them. I also equipped extra clips, helps to avoid favorite weapon chasing.
And last but not least... it took a fair amount of dumb luck. Don't count on your fellow aliens, the most they can do is finish off humans you already wounded heavily. I often got more points from suicides than from their kills.
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