I've tried considering a "minimum" number of aliens killed before (see post of minimum kills here
) but let's try another approach.
In Aliens, there could be as many as roughly 150 xenomorphs (high estimate if we consider nearly every colonist to have been incubated and not killed before) and for simplicity, let's ignore the one chestburster death and the queen in these numbers. There's roughly 3 scenes where there could be mass numbers of xenomorphs dying (going by the director's cut), and since we have no evidence to suggest otherwise, let's assume each scene averages to have all of the xenomorphs die (unlikely, but let's go with it) so that'll make roughly 50 xenomorphs per encounter. At the start of the first encounter, there are 10 Marines that would be engaging in the combat, 4 sentry guns in the second encounter, and 4 combatants I believe (Hudson, Hicks, Vasquez, Gorman) in the third (Burke and Newt don't contribute, and Ripley manages 1 but I don't think is seriously contributing).
So, at absolute best
, a marine/sentry gun takes down roughly 12-13 xenomorphs in the final encounter before unable to kill anymore (dying, running out of ammo, retreating). At absolute worst (based on my minimum post linked), a collective 10 marines are able to take down 4 adult xenomorphs (so a little under half a xenomorph per combatant) in the first encounter before being out of the encounter (and they were retreating fairly soon in that encounter).
If you want to go by the theatrical version, the two encounters would have roughly 75 xenomorphs each, and a best then becomes 18-19 xenomorphs killed per combatant. If you want to take the absolute illogical extreme and say only the bare minimum of adult xenomorphs died before the final encounter (4) and nearly all the xenomorphs die in the final encounter (146), that makes 36-37 per kill. Anyone thinking more could be killed with the weapons they had I think has gone far past the evidence shown, especially as this "maximum kills" exercise ignores how there are still a non-trivial number of xenomorphs alive and tracked after that final encounter (when Hicks and Ripley initially try to get Newt when she got seperated).
Take that information as you will. I'm sure someone could probably math the average kill ratio before death in the ARPG system, but that person is not me.