I know it's mentioned in the Anglo Saxon Chronicle but it seems inconceivable that a single man could hold back an army of 15000 at all. How likely is it that it was a small group of guards using a shield wall and the story was later embellished?
I searched previous questions but it didn't provide much of an answer.
Short answer: We have no idea.
Longer answer: We really have no idea.
There's no actual evidence that the single warrior at Stamford existed, as the ASC and Henry of Huntingdon are the only mentions of it - Heimskringla doesn't, so far as I recall, mention the single warrior at all.
As far as it possibly being a small group of soldiers that got embellished into a single warrior goes, possibly, but again, we have no real way of knowing for sure.
I've had a look around, and there are some details missing which might add or detract from the posibility of the lone viking legend being true.
The Anglo Saxon Chronicle states that he managed to cut down around 40 men before a lone soldier floated under the Bridge and struck him from between the planks with a long spear.
"Then was there one of the Norwegians who withstood the English folk " that they might not pass over the bridge nor obtain the victory. Then an Englishman shot with an arrow, but it availed naught; and then came another under the bridge and pierced him through in under the bymy."
Note that this account does not tell us the width of the bridge, or the manner of men he is reported to have fought. If said Viking was an experianced warrior in Maile, with a stout shield a good steel sword it is possible he could hold a force many times his number at bay, perhaps slowly retreating down the bridge.
As you say, there is perhaps an element of embelishment, but the tale is not that he held the entire Saxon Fyrd by himself, simply that he stalled them while the Norse formed up and made ready.