Why is everybody forgetting that he's weak when exposed to kryptonite or are we just going to pretend that this won't be the case and that he's just going to lose to Ultraman and the Engineer for no reason other than being weak?

Those scenes are probably going to be from one fight scene since they're not giving away the entire plot or story in the previews.

What if those scenes of him being weak are from one fight, one continuous scene? I'm going to guess this was one fight where Superman was exposed to kryptonite and won't be the entire movie. Superman is extremely weak when exposed to kryptonite and yet all I keep reading is that Superman has no weaknesses.

But.... he does. Kryptonite and magic can make him weak enough to be defeated. And I'm guessing this is from one fight, not the entire movie. But we're so used to previews giving away the entire movie in DC that everybody is assuming this to be true.