I watched some video of attitude era mode, and most of the hidden objectives seemed to be obvious and/or unavoidable... I think they're just "hidden" to not ruin the story/twists for people unfamiliar with the attitude era before the match begins... not as an actual challenge to discover them.
Like, before the match it will say, "hidden objective" but then halfway through the match a cutscene plays and then it tells you "use the sharpshooter on your opponent in the ring" ... and that's the hidden objective.
Seems to work that way in the videos I've watched. But I don't have the game yet, so not positive.