Turin wasn't alone, he was helped by Iluvatar. This fight wasn't a sword fight, it was mystical. It was Turin's fate to kill Morgoth. For beginners, Turin was dead and the Valar did not have the power to bring him back to life. Turin was being hurt when approaching Morgoth, if he was still the old Turin he would have died, but Iluvatar protected him.Well you are missing something. Yes, Tulkas fights Melkor in Dagor Dagorath. But after Melkor defeats Tulkas, Turin fights Melkor one on one. Then Turin kills Melkor. In other words, Turin does not kill Melkor with help. Because before the fight between Turin and Melkor, Melkor already defeats Tulkas. And then Turin fights Melkor one on one. There are even sources and visuals that show Dagor Dagorath is like this.
"It is said that in the Dagor Dagorath, Tulkas will once more oppose Melkor, and will directly fight him in the battle. The victor is unknown, But probably Melkor, For it will be Túrin who will slay Melkor."![]()
Tulkas
Tulkas was a Vala. He was the most warlike of the Valar and the spouse of Nessa.tolkiengateway.net
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Also, about Morgoth defeating Tulkas, in the past Tulkas had easily beaten Morgoth, so either Morgoth was a lot more powerful (which will make him killed by a human even less likely) or Tulkas much weaker, or perhaps it was Iluvatar's will that Tulkas and Eonwe wouldn't win against Morgoth allowing Turin to give the final blow.

