Who were the heroes in the battle of Act 1 Scene 2?
The battle in question was against the Norwegian king and his forces, led by Macdonwald. Macbeth and Banquo, both generals, led the charge against him.
The unnamed Sergeant, being questioned about how the battle is going, tells King Duncan that Macbeth "disdains fortune"--that is, he doesn't care what fate may lie in store for him--and chopped and sliced his way through Macdonwald's men "Till he faced the slave." He had not words or respect for him, but did not stop fighting until he'd "unseam'd him from the nave to the chaps"--cut him open from the navel to the jaw--and put his head on the battlements.
No sooner had he defeated Macdonwald than fresh men and horses arrived for the enemy. Macbeth and Banquo, despite being tired from the battle they'd just won, waded into the fray. The Sergeant reports that they were "As cannons overcharged with double cracks, so they / Doubly redoubled strokes upon the foe," as though they meant to die or create another Golgotha. Once again, they face their enemies without thought to their own well-being.
The heroes of this scene are Macbeth and Banquo. According to eyewitness reports, they acted with incredible bravery in the battle against the treacherous thane of Cawdor and the king of Norway.
Macbeth and Banquo were so fearless on the field that the Captain compares them to "eagles" and "lions," animals of considerable power and bravery. He also compares them to "cannons" loaded with extra ammunition. Moreover, the Captain wonders if they wanted to make this battlefield as memorable and significant as "Golgotha," the site of Jesus's crucifixion.
As for Macbeth, he is compared to "Bellona's bridegroom." Bellona was the Roman goddess of war, so by making this comparison, Macbeth is singled out for his bravery and skills in battle.
It is his actions against these enemies which lead Duncan to reward him with the title, thane of Cawdor. This reward, however, only serves to drive Macbeth's ambition for the crown.