In general, whatever your weapon is, always aim for the head. Even the triple tap secondary fire on the pistol is effective if you aim for the head. The one exception to this is the Combat Androids who function quite well without heads. For more on the arms check out our Aliens vs Predator Weapon Guide.

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Aliens
You’ll encounter Aliens in the Marine Campaign that are perfectly camouflaged on the wall so always proceed with caution in the Hive. Remember that your Motion Tracker has no indication of height so the approaching enemy could be above or below you. When Aliens approach it is best to back pedal and fire. If they charge you then try to side step when they get close. Shoot any explosive canisters or tanks in the area to damage Aliens as they pass by.
In multiplayer it is even tougher to fight against Aliens and you’ll find they frequently flank you and pull off kills from behind. Fight back to back with your team mates or against a wall to avoid being blindsided. Co-ordinating fire with a friend helps you out down Xenomorphs fast.
Predators
Seeing Predators before it is too late is your big challenge here. We’re all familiar with the cloaking device effect so if you see any rippling ahead then fire. You can also watch out for the laser sight which precedes a Plasma Caster blast. You won’t have much Predator contact in the campaign but you’ll come up against them frequently in multiplayer.
Predators are a great deal tougher than you in melee combat so keep your distance and aim for the head. If they do get in close then the Shotgun is your best bet. If you can keep them at range then use the Scoped Rifle. Remember that they can heal themselves so if you do some damage - don’t let them escape - follow it up immediately and finish them off.
Combat Androids
You’ll encounter these guys in the campaign and they can be a pain. Since they can operate without heads you should aim for the legs to put them down and don’t stop firing until they’re neutralised. Once dead they are liable to explode so keep your distance. The best weapons for spotting and despatching them are the Scoped Rifle or if you get up close use the Shotgun.