It wouldn't be a tough fight at all. Hal or Kyle could pretty much solo the whole DB Universe so could Wally or Bart if they really wanted. It would be hard for the DBU to do anything when a Flash takes away all their speed so they can't move, adds it to his own speed, and then hits them with punches that could KO Superman. As for a Green Lantern they have pretty much an endless supply of what they could do.