doesn't abandon the concept that aliens are good at ambushing prey. That's how the aliens wipe out a large portion of the CM detachment in the reactor chamber. However, ambushing is a tactic heavily dependant on the environment and the enemy's level of caution. It simply doesn't work in a lot of situations. Enemy won't always blunder into locations suitable for a xeno ambush.
Defending against ambush isn't that hard with the correct knowledge, especially with a vision mode that highlights aliens in bright green against a dark background. For an experienced xeno-hunter, Wolf is really oblivious to danger. For instance, early on he fails to spot the alien on the roof. One would think that he wouldn't miss the big green blob hanging from the ceiling, but he does...just as the national guard dude misses the alien next to the car. It's just bad writing.
For aliens vs. "advanced species with powerful guns" to be balanced, the writer would have to employ plot devices such as:
1) Colonial Marines have no experience with Ripley's aliens
2) Amateur Gorman lets his men blunder into the reactor chamber, Sarge collects ammo
3) Pesky humans steal the predators' shoulder cannons
4) Predators and aliens fight inside a cramped pyramid (perfect environment for wall-crawling aliens)
It's not melee combat so much as the Alien forcing the protagonist into close quarters.
The aliens didn't really force Wolf to go into the sewers. Going into the sewers was Wolf's attempt at keeping his mission covert...rather silly in light of how things turned out (he strung up a skinned dude, aliens ended up infesting the entire town)
Can't imagine you'd ever catch them running straight towards you over open ground.
Quite easy really...
1) when predators destroy the environment where the aliens are trying to hide, flushing out the aliens
2) when the queen realises the preds could just blow up the hive and orders her drones to attack
and perhaps the easiest example...
3) when predators decide to breed aliens in an evironment that doesn't favour the aliens. Instead of a pyramid, they could fight them in their version of the Colosseum
Aliens use cover. Great. Predators can blow up that cover. Aliens can't really force predators to fight on their terms. Predators choose to fight on their terms (i.e. ritual hunting or unwillingness to draw too much attention).
When predators go all out (not really all out but close enough), what happens is something similar to the first few pages of the AvP War comic
EDIT: Forgot to post the pics