"I can already tell you that some of the things Childermass did with the cards, he couldn't do beforehand. As for whether they're the source of his Harbinger magic or simply a focus, I don't know. And I'm afraid the only way either of us would be able to tell is to fight him again."

Which Strange doesn't look too pleased about. He knows he's probably going to end up fighting Childermass at some point or another but he really doesn't want to. Unsurprisingly, Strange isn't taking well to the idea that he'd get his ass whooped (again) by Childermass (again).