On the forum I read that if you kill all enemy models (in two-player game) you will win. I do not know if this is a good rule. For example, in mission 1 (marines vs predators) predators can easily kill marines at a distance and do not collect trophies or do not care about other victory condistions.
How in mission 1 (aliens vs predators) aliens have to kill 5 models if there are only 2 predators?
We also played marines vs aliens and I beat the marines before completing the remaining victory conditions. Then what? Win or lose?
The campaign missions are designed for 3 people. That means, there are 2 Predators (count as 4 models) and 5 Marines for a total of 9 potential Targets for the Aliens. However, if you play only 1v1 and the opponent has less models than what you are required to kill, then treat the mission as saying "kill all of your opponent's models".
The Predators can indeed shoot the Marines, Hot Plasma is a hard thing. Do you use Tactical Move though? The Preds can fall too, especially against that pesky Smartgun.