Since Aliens have to get in close to kill their prey the approach to each type of enemy is exactly the same. Use speed and stealth to get as close as possible without being spotted. You can target a victim with Left Shift and use Space to pounce on them but you’ll have to follow up fast to get the kill.

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The easiest way to despatch all three targets is to sneak close enough to pull off a devastating gory kill. The prompt to press E will pop up when you get close enough.
Marines and Combat Androids
Always stick to the shadows and approach enemies by running on the walls or, even better, the ceiling. If they don’t see you coming then you’ve got a great chance of getting the kill. The weaponry of the Marines will tear you up fast but you can regenerate health over time so if you are spotted at distance then run away, heal up and plan a new attack. Charging into gunfire will result in a quick death. If you can get in behind them then both enemies can be killed instantly with the E attack.
Predators
When you’re up against Predators you’ll have to use your superior speed. Since they can also fight well hand to hand, or blade to claw, you don’t really want to go toe to toe with them. Zoom in and swipe and then swing round for another attack, don’t get bogged down. Once the Predator is seriously hurt you can attempt the sneaky kill with E.