What i do is, create 3 maybe 4 teams of aliens. I send the first team of aliens with about 6, 7 aliens. Then a team of like 5 aliens after the first group. I send team 1 in to attack the preds. While they are fighting the preds, i send team 2 in and they take out the preds while they are busy with team 1. Then, i bring a 3rd team in to finish them.
Sometimes i send a group of 10 aliens in, and 5 maybe 6 facehuggers behind them. While the preds or marines are busy with the aliens, the huggers come up from behind my aliens and Jump the marines or preds. The game is Pure strategy, you just got to know how to do it. But it works well for me.