There are decapitations aplenty in this movie, with many lightweight weapons, short swords and small axes. Yet not even executioners during medieval times were always able to chop off a head with one stroke, even though the criminal was resting his neck on a chopping block and holding perfectly still and the executioner using a very heavy axe.
Quintus Dias is not a possible roman Name. Quintus would be a possible pronomen, but there's missing the nomen gentile (family name, ever with an "-us"-Ending), every Roman had. Dias would be a strange cognomen (third part of roman names). Example: Caius Iulius Caesar = Quintus XXXXX Dias.
The spear the IX Legion carries is a period-correct piece of Roman military equipment, known as the hasta. While certainly less well-known than the pilum (plural pila) throwing spear, the hasta would be a better tactical choice for a legion marching into heavy forest where the space required to bring up and throw a pilum properly is not guaranteed. The hasta, being basically a "typical" spear, is a more versatile weapon. It should be noted that every legionary was issued both a pilum and a hasta, though the pilum featured more prominently in the Romans' preferred open-field strategies.