I like the idea of a "where are they?!" "The motion tracker says they're right on top of us!" scene except instead of motion trackers, it's the dogs barking with furious terror at seemingly empty darkness while the uneasy soldiers look around everywhere in alarm... everywhere but directly above in the treetops.

Gladiator vs Alien. Clashing this Summer!

I think if the xenomorph doesn't have to offer battle, if the legionaries are unprepared, or if the combat area is something like 'the woods' or 'a town', the legionaries are screwed, end of story. Any of those is simply too much of a disadvantage for the legionaries to overcome.

If it's someplace relatively open, plains or hills, or if the Romans are given an opportunity to make a little defensible position, then the terrain isn't fight-ending.

If the century is given time to prepare, then they'll come stocked up on javelins. As in, more than usual. This is really the only way they have any sort of a chance against the xenomorph - the gladius is too short (and a xenomorph's reach too long) to present an effective threat, and their shields are unlikely to hold up any better than the metal environments xenomorphs regularly tear up. So the typical tactic of 'close the distance aggressively and stab them up close' is unlikely to have favorable results, especially considering the gangly proportions of a xenomorph. You could compare the reach of a xenomorph to the pikes of a sarissa phalanx, which the Romans also had trouble 'closing in' with. But unlike a sarissa phalanx, a xenomorph has no sluggishness in changing direction or position.

The Romans would have to carry enough javelins with them for multiple volleys and opportunistic shots, rather than the normal 2-3. Likewise, they would have to fight in a looser order, like skirmishers. The mass of their formation isn't going to mean jack shit to a xenomorph, except more targets to hit at once with claws, tail, and acid.

A scorpio wouldn't even the odds if the xenomorph acts like it does in Alien - like an intelligent, demonic being, full of understanding and malice. It would just tear it up at the first opportunity. However, if the xenomorph acts in a more animalistic way, like in Alien^3, or with a blind, insectoid aggression, like in Aliens, the scorpio is a major game changer. Scorpiones manned by skilled crews were terrifyingly accurate, with Caesar relating a story of a single scorpio plinking men on the exact same spot on the wall, shot after shot, from a pretty good distance away during a siege in the Gallic Wars. In this case, the legionaries would be best used to corral the xenomorph, by throwing javelins and using them as crude thrusting tools, so the scorpio can line up a shot. Or several shots, rather, because it's unlikely to be killed with just one.

Otherwise, the legionaries will have to volley the xenomorph's general direction with javelins, as the reflexes of a healthy xenomorph means that one throw, no matter how strong the arm or how accurate the aim, is unlikely to hit; only by filling the entire area with missiles can a hit be secured. And a few pila are unlikely to end the xenomorph. But as the xenomorphs wounds mount, and its speed and ferocity diminish, it becomes easier to hit with missiles, and from there, it's just a question of endurance - who wears out first? The century's rapidly diminishing manpower, or the xenomorph?