Caesar likes to point out the occasional act of heroism and although one should expect him to embellish somewhat it seems likely that his stories are broadly factual (given the overall character of the work). He recounts the following event in De bello Gallico V 44:

In that legion there were two very brave men, centurions, who were now approaching the first ranks, T. Pulfio, and L. Varenus. These used to have continual disputes between them which of them should be preferred, and every year used to contend for promotion with the utmost animosity. When the fight was going on most vigorously before the fortifications, Pulfio, one of them, says, "Why do you hesitate, Varenus? or what [better] opportunity of signalizing your valor do you seek? This very day shall decide our disputes." When he had uttered these words, he proceeds beyond the fortifications, and rushes on that part of the enemy which appeared the thickest. Nor does Varenus remain within the rampart, but respecting the high opinion of all, follows close after. Then, when an inconsiderable space intervened, Pulfio throws his javelin at the enemy, and pierces one of the multitude who was running up, and while the latter was wounded and slain, the enemy cover him with their shields, and all throw their weapons at the other and afford him no opportunity of retreating. The shield of Pulfio is pierced and a javelin is fastened in his belt. This circumstance turns aside his scabbard and obstructs his right hand when attempting to draw his sword: the enemy crowd around him when [thus] embarrassed. His rival runs up to him and succors him in this emergency. Immediately the whole host turn from Pulfio to him, supposing the other to be pierced through by the javelin. Varenus rushes on briskly with his sword and carries on the combat hand to hand, and having slain one man, for a short time drove back the rest: while he urges on too eagerly, slipping into a hollow, he fell. To him, in his turn, when surrounded, Pulfio brings relief; and both having slain a great number, retreat into the fortifications amid the highest applause. Fortune so dealt with both in this rivalry and conflict, that the one competitor was a succor and a safeguard to the other, nor could it be determined which of the two appeared worthy of being preferred to the other.

(translation by McDevitte & Bohn)

It depends on what you consider to be 'reliable,' really. Source criticism is a huge part of dealing with primary and secondary sources, and a lot of things that seem implausible are added for literary effect.

For example, Njáls saga talks about Gunnar Hámundarson being able to use a sword with both hands equally well, leap the height of a man while wearing his full suit of armour, and describes how he kills a half dozen men and wounds twice that number while defending his house during his last stand. Probably not accurate, historically, even though we do have records of some of the events in Njála occurring.

Egils saga is another one where we have people described as hacking their way through formations of soldiers single-handedly and performing all sorts of super-heroic acts in battle. And we do have records of those battles occurring in other sources, but not in such a way that describes the individual characters hacking through men like farmers through wheat.

You have to remember that a lot of primary sources in the medieval period were written for specific purposes; sagas were meant to increase the status of families or kings, and so actions happen in them that seem implausible, and may never have occurred. Unfortunately, they're oftentimes the only narratives we have of those events, and corroboration comes from annals and chronicles, which might just be a footnote or - if they involved a king or someone who was made a saint - poetic interludes describing actions in terms that are just as unlikely.

You're talking about a period of time where it was often the difference between a professional soldier and the local militia man who was likely going to go home and farm once the summer campaign season was over.

Were there skilled soldiers? Absolutely. Obviously something like 300 or Troy is a heavy dramatization but there are plenty of examples of situations where an army is outnumbered by still manages to beat the pants off a larger force for any number of reasons, none the least including troop training and skill. Just the idea of a battle like (yes, I am aware this isn't ancient world theaters we're talking about, bear with me) Agincourt should speak to the quality of the soldier when they don't try their luck surrendering or just running away instead of standing and fighting fairly significant odds.

It'd help if you had a more precise question- EG: were Spartan soldiers really as effective as presented, what about Persian Immortals, Mongol horse cavalry, Egyptians chariots, ectectect.

Part of the issue with translating a lot of old texts is that by the time it is translated to whatever you're reading it in, it has probably been translated from a translation from a translation, context sensitive portions can be a bit dubious and it can be hard to sort between academic license and simple, rote fact.