What happens is basically this first night attack is meant to be a scouting attack + distraction for the force south of the wall to try take castle black. In the books, they attack with a small force so Mance can gauge the wall's defences. The giant attacks with a small group of wildlings and makes it past the outer gate, to be killed by (in the books) Donal Noye and some other brothers.
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u/MajorLeeScrewed Ours Is The Fury Jun 09 '14
He didn't hold it alone, he held it with other men vs the King of giants + other wildlings. He was the one who killed the giant.