If I control 1 battlefield then hide a card. then on my next turn i attack an enemy battlefield. can i show the hidden card. for example evelynn and bring a unit over from the battlefield I am now attacking

Yes. You can play Evelynn from Hidden during the attack, and her trigger can pull a unit from the battlefield you're attacking.

Details:

• Hidden cards gain Reaction (Rule 811.6), so you can play Evelynn while the Combat/Showdown is happening at the other battlefield — hiding doesn't lock her to your own turn's Main Phase, and hidden cards can be activated during a showdown elsewhere (FAQ 7757).
• But Evelynn must be played to the battlefield where she was hidden — your controlled one, not the contested one (Rule 811.1.d.1).
• Her trigger says "move an enemy unit at a different location," which is exactly the Tideturner-style exception: because the ability explicitly requires a location that can never be at her own battlefield, she may choose her target freely (Rule 811.1.d.2). So yes, she pulls an enemy unit over from the battlefield you're attacking.