Have you played the Metroid series? You could use some of the elements of Phazon or the X parasites.

metroid.wikia. com/wiki/Phazon
metroid.wikia. com/wiki/X_Parasite

Off the top of my head, you could Include Phazon in the story as Samus’s plan to take out the aliens. After the events of Metroid prime 3, she is able to access a final remainder of it for a situation like this and can use it to take out the Xenomorphs, either using it on herself for a massive power up (at the cost of maybe dying or being corrupted), or to poison and take out a large horde of them at once (but at the cost of it either mutating them into a more vicious form if they are able to resist it).

Lara could be used as bait to lure them to a location in which Samus takes them out with a Phazon bomb or her going into “Hypermode” to go ham on all of them.

The climax could be that the plan generally works but something goes horribly wrong like…

1. Samus takes them ALL out with hypermode but gets corrupted. Goes insane and turns on Lara. Thus the climax is Samus vs Lara. One of them dies.
2. Samus takes them out with the bomb but Lara gets caught up and gets corrupted. Same thing happens, Samus vs Lara.
3. Samus takes them out with the bomb, but one of the Aliens resists is and gets corrupted. It goes gains a new power and basically becomes the final boss for Samus and Lara. It wrecks one of them and the other manages to succeed with he sacrifice of the other.
4. The plan goes perfectly…but the alien queen appears and is the true final boss. Samus is Drained, Lara is fucked up from being bait, they both go out in one final effort to defeat the queen.


If one of them manages to make it out alive, then you totally at the end must have her think its all over…only to suddenly be victim to a chest burster or something.


Not sure about the X parasite however, but with it being able to clone a fucked up but more powerful version of stuff like the SA-X, you could think of something pretty cool.