Bran knew what was going to happen the entire time. He returned to Winterfell. He gave Arya the dagger. He concocted the plan to lure the Night King to him, but everyone believed he would be lured there and they would fight him. There was no assassination plan. Even Arya didn't think about doing that until Melisandre picked up on what was happening and suggested it to her. The Night King is super intelligent, and seems to know what people are thinking, their intentions, and has extreme foresight into what is going on, but he is not an all knowing, all powerful being. He still comes from a human, and has arrogance, which we see it with the smirk. Bran worging seemed to both provoke the Night King, and to hide his true intentions. If anyone can hide what is happening, it is Bran. The Night King didn't move in until everyone was disabled, dying and they were obliterating Winterfell. He should have waited longer, but again, he has arrogance. Bran even said everyone had a purpose, and knew the Night King would not risk himself until he was on the very brink of victory. That is why him and the white walkers do not move in until their army of the dead takes out the majority of the threat. Arya literally trained the entire series for this moment. If Bran told her to wait in the bushes and jump out, and everyone else was in on it, you think that would work? I don't think anyone could outfight or outsmart the Night King, except Bran. Without him, his connection to the Night King, his knowledge, his purpose, Winterfell would have just been obliterated. So when people say Bran wasn't doing anything, I just think they totally miss what this show has been leading to. Maybe it isn't as elegant and masterfully laid out as it could have been, but they have been hinting at everything for quite a while.