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.