Can I use a Hidden Tide Turner from another battle field to react to save my unit on the attacking battle field?

Yes, you can use a hidden Tideturner from another battlefield to swap with a unit on the battlefield where combat is occurring.

Ruling

According to FAQ #8297 and FAQ #7907, Tideturner is an exception to the general rule that hidden cards must target units at their own battlefield. Because its ability explicitly requires targeting a unit at "another location," it is permitted to choose targets freely from among available options.

Sequence of Events

1. Playing from Hidden: You play Tideturner from its facedown state as a Reaction. This action opens a Chain (Rule 737.1.c.3).
2. Entering Play: Tideturner enters play at its original battlefield.
3. Triggered Ability: Tideturner’s "When you play me" ability triggers and is placed on the Chain as a Pending Item (Rule 376.3).
4. Targeting: As the triggered ability is finalized on the Chain, you choose the target unit at the other battlefield (Rule 352.4.b, FAQ #7907).
5. Resolution: Once the Chain resolves, the swap occurs: Tideturner moves to the target's location, and the target unit moves to Tideturner's original location.

Addressing Your Concerns

• Timing: You do not need to wait for a specific game event to "respond" to; you can play Tideturner as a Reaction at any time you have priority during the Showdown, provided the Showdown has not yet ended (FAQ #802).
• Targeting: Targeting for Tideturner's "When you play me" ability occurs when the triggered ability is placed on the Chain and finalized, not when the ability resolves (FAQ #7907).
• Location: As confirmed by FAQ #8297, you can activate a hidden Tideturner from one battlefield to target a unit at a different battlefield because the card's text explicitly allows it, overriding the standard hidden targeting restriction (Rule 737.1.d.2).