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[Eagle of Fire]

I have never reloaded a game because I could not find an ennemy.

The thing is, you need to look in places you would not think about finding an ennemy. Like a hole in the deck of a ship trapping a NightOne, or an ennemy which spawned behind walls... That's when it is time to prime grenades and go Alien hunting.

Don't forget, Aliens usually always spawn at or around the same place...

[Saccade]

Or the "level display" thing makes them invisible, unless you're on that level...

Sometimes you have to choose to display all the different heights, as a disc or floater will be hovering (having run away) in some hidden corner that is not accessible by yr troops.
One that's not somewhere they can walk to, like they can on roofs or in buildings - shooting out the windows.

Then it's just one shot to take out the crippled foe and "WIN!".