My opponent has one unit at a battlefield and hidden card (tideturner). I have one unit in base and I play void assault, moving my unit from base to battlefield and opponent's unit to his base. Does he have to flip hidden tideturner in response to the void assault or can he do it after void assault resolves, so he can swap places with the unit in his base?

Wrong ruling

He must react with the hidden card before Void Assault resolves, otherwise he will have lost control of the Battlefield and the hidden card will have been trashed. As per the rules of Cleanups, there is an order to the outstanding tasks that occur (rule 323). Task 4 checks for loss of control. Task 5 removes hidden cards. Task 6 marks a showdown staged. So respond before the resolution of Void Assault. Play Tide Turner to the Battlefield and not use his ability. Or do not respond and loose the hidden Tide Turner in the cleanup following Void Assault.