So I got to thinking about xenomorph armies (like the xenomorphs from the Alien franchise, or [spoiler alert] the white-spikes from "the tomorrow war") on the way to work this morning and realized that they'd be so much more effective paired with three other weapons deployed simultaneously. A plague tailored to the dominant intelligent species to distract and weaken them would do nicely. The xenomorphs can of course distract them militarily and wipe out the planets large fauna for food. A locust or fungus sort of organism that consumes the planets vegetation and small fauna would eliminate any agriculture, and a leech/crab like aquatic organism that spreads to reefs and up rivers and lakes by latching onto any large organisms like a remora does and then spreads in every densely populated ecosystem would eliminate the planets major marine ecosystems. Combined, and tailored to survive that particular planets biology or survive on any organic chemistry (like, these four weapons are all adaptable to any environment with liquid water and carbon-based life and can consume any living thing, somehow) you could wipe out an entire biosphere and any intelligences dependent on it and unable to escape in only a decade or two. How would you divert your society to fighting four separate problems simultaneously? If you survived the plague and killed the xenomorphs, you still face an extinction level event from the invasive all-consuming nightmare locusts and crawdads.
It would be just as easy, and probably easier to clean up afterwards, if the aliens just punted a few asteroids/comets at the planet. A pre space civilization like ours would be totally defenseless, and also unable to prove the impacters were sent our way by alien intelligences. They'd just show up on the radar one day and we'd be fucked, not enough left of them afterwards to tell how they ended up on that course, assuming anyone smart enough to figure that out survived.