The film itself has been criticised a lot for it's inaccuracy. The opening battle is no exception.

The use of fire arrows and heavy artillery like onagers is more of Hollywood fantasy than based on historical fact. While both Caesar and Pliny do in fact record the use of flaming arrows they would be much more common in siege battles where buildings could be set alight. The same goes for the onagers being used mostly in sieges against walls, although the use of ballistae and the smaller scorpion would be more commonly employed in non-siege warfare.

The armour of the legionaries is pretty accurate being lorica segmentata and there is good attention to detail with the archers wearing lorica hamata as well as a more elongated helmet - commonly associated with Syrian auxiliary archers who have been recorded as far north as Hadrian's wall and depicted in a similar way to the film in reliefs. There also appears to be differentiation between the shorter gladius the infantry soldiers use and the longer cavalry sword the spatha. However, the helmets worn by the legionaries only barely resemble the Imperial Gallic style helmets that would have been in use at the time.

The battle itself is very questionable. The Romans advance with their pila yet are not shown to throw them at all. Indeed, Roman soliders would in fact carry a heavy pila and a lighter one and the effect of these would have been devastating to a charge. The Roman formation is also incredibly thin, and the charge of the 'barbarians' is again Hollywood fantasy. Indeed, the Marcomanni (the tribe who presumably are depicted here) wear no armour and dress in 'barbaric' clothes when in fact they were at the time capable warriors who fought with spears in a stable battle line, not just running straight at the enemy. The fight quickly devolves into a series of one on one battles rather than containing any sort of the formation fighting both sides were capable of and, in the Roman's case, famous for.

There are some other minor points such as the Roman cavalry being depicted with stirrups which they would not have had at the time. Furthermore the battle only shows what appears to be Roman legionaries, there would have been auxiliary units apart from archers present, more specifically the cavalry as the Romans were not known for their cavalry. And while the movie makers want to make it obvious who's on the Roman side by making them all wear the same armour there was variation amongst the Roman troops ranging from the segmented armour shown in the film (lorica segmentata), to chain mail (lorica hamata) and even armour made out of metal scales (lorica squamata). Finally we see the Emperor surrounded by the aquillae (Legionary Eagles). In fact they served as important rallying points during a battle as to lose an eagle to the enemy was considered the greatest dishonour and troops would fight much more fiercely to protect it. You would expect the aquiliferi (standard bearers) to therefore be placed in the battle line at some point to prevent routs and inspire the men. One example of this is from Caesar's De Bello Gallico where he tells us how his troops would not leave the boats and go onto the beach where the terrifying British troops waited. The aquilifer jumped over the side and the men then rushed to protect him therefore beginning the battle all because the eagle could not be lost.

TLDR: Looks impressive but is not very accurate at all.