In my opinion, they should be hard to kill, but not damn bullet sponges like in AFE, especially if you turn up the difficulty. There's nothing I hate more than bullet sponges cause how unrealistic they are. Studying todays rifles, I realized we have some crazy powerful stuff already. Aliens should tank a few Pulse Rifle bullets, a burst or two. The old Perry novels went the opposite way and literally had the Aliens dying and exploding after one bullet which I also hated.
Humans under adrenalin can tank a lot of bullets from pistols if they have enough mass and vital organs aren't hit. So scale that up to an Alien, they could take a few bursts of rifle burst. But nothing can survive explosive tipped rifle bullets. You'd have to have extremely tough armor, and be big and thick. So nothing less than a Crusher, Praetorian or Queen maybe could survive 30 direct hits of a Pulse Rifle. That would be unrealistic.
The danger from the Aliens comes from their speed paired with a tough exoskeleton and underutilized acid blood defense. They don't need to be shown as unstoppable bullet sponges (like in AFE), just smarter and faster (like in AVP2010). They should always try to outflank the enemy in new and creative ways. Climb walls and ceilings more. They win the numbers game. They quickly create new offspring that is battle ready in less than a day, and don't require a lot of resources.
Equipping marines, guns and training personnel? A logistics nightmare when you have a fast spawning enemy born battle ready. Marines have the firepower advantage using big big guns, but if the Aliens' behavior would get inspired by the resourceful bugs that people keep on disrespecting, there would always be one left somewhere. Hiding. Waiting. And starting again anew. All you need is one for the infestation to restart. That's the true horror for me.
And why do people hate bugs so much? Or see them as dumb? I get that comparing Aliens to bugs is silly in the derogatory sense, but Alien has always had insectile inspirations (impregnating wasp). Ants and termites are extremely fascinating, just read their wiki page for starters and you'd see how awesome they are. I don't believe that just cause the Aliens are a part of a hive, they have to be dumbed down. They can be cunning, problem solving and relentless, plus be part of a hive. Literally mix ants, termites, crazy loner wasps, and JP's portrayal of Velociraptors and you get the Alien.
I never thought of the creatures in Aliens as dumbed down, stupid, weak and "just bugs". I thought they were even more of a threat than in the first movie since they kicked the marines asses in every encounter. I was impressed and fell in love with the creature after that.