Is there secret when you melee attck when you are using the Pred. or the Alien?

When in close combat as an Alien or Pred just watch your opponent.

When facing Marines, close combat is a pain. Most will just unload on you, but they can block your light attacks (RB) and counter with a melee bash of their own. If you can try to land a light hit followed by a Heavy attack (LB)

When facing Predators, watch their movements. Predators light attacks are what you should always be ready for by blocking during the fight (LB+RB). Predators block by holding their claws in front of their face, when you see this DO NOT use a Light Attack, they will block and counter. Instead, get just out of range of a quick Light Attack and use a Heavy attack, then another, then a couple of Light Attacks or you can press X to use a Front Grab. A predator's heavy attack is shown by it holding its right claw back in a charge, this is also accompanied with a loud growl. To counter this use a Light Attack as soon as possible, DO NOT block as a heavy attack will tear thru it.

Aliens are the same concept as Predators except for the looks. Aliens block by holding their claws in front of their face. And their heavy attack shows them standing up, with their tail arced around them ready to strike. Watch your opponent at all times, Aliens are great at getting around their enemy for a grapple.