There is a story that a giant Norse axeman singlehandedly held up the entire English army and killed at least 40 Englishmen at the Battle of Stamford Bridge. Is this story true or just a myth?

It's kind of mentioned in some accounts. Beyond that, we don't know. More can always be said on the matter; for the meantime, that particular maybe-fictitious Dane has been asked about before, so here's a few previous threads about him:

u/y_sengaku examines what our sources are on the matter;
while u/wotan_weevil and u/sagathain look at how plausible it is, in real life and in literature.