Blackbeard could take out Crocodile or Hancock or Moria by himself if he had too, I don't think that's a problem.
Luffy was lucky that Ace was his brother, as Blackbeard said, when Ace learned that Blackbeard was after Luffy he couldn't turn back from the fight.
As for Impel Down, Blackbeard helped Luffy and Jinbe as well.
He also removed the poison that stopped Buggy and other escapees in Impel Down.
And he also destroyed the guards at the front gate before entering to Impel Down.
Then they managed to pass the gates of justice thanks to Blackbeard;
They both helped each other.