^Agreed. In a no cover situation, it all depends on how lucky you are, and whoever sees whoever first most likely will win.
If there's no cover, you could always create your own, i.e. vehicles, riot shields (lol COD), or any sort of improvised cover you can think of, and have your guys attack the vulnerable enemies while y'all are behind cover.
You could also try to draw the enemies into an area with cover. Hide most of your team in the aread with cover, send a few guys out to engage the enemies in the flat land. After getting the enemies' attention, have your team retreat into the cover. If the enemy follows them into the cover, where most of your team is, just surround and kill.