Can hidden here to help play Baron? If so is it played to the battlefield or baron pit?

Yes — and Baron enters the Baron Pit, not the battlefield where Here to Help was hidden.

Here to Help's "play a unit from hand to a battlefield you control" is a play effect, so per Rule 811.1.d.3 the unit it plays has to be played to the battlefield Here to Help was hidden at (the one you control). But that location choice is only where he's played to; Baron's own ability is a replacement effect that replaces him entering that location with him entering the Baron Pit if it was created instead (confirmed in FAQ #10250). So the destination gets rewritten to the Pit and Baron never actually enters the hidden battlefield.

Everything else follows normally: you still pay Baron's cost reduced by [3], the Pit becomes contested by your unit, it triggers a Non-Combat Showdown, and you conquer it after the showdown ends if you haven't already scored it that turn. Baron must be in hand — Here to Help can't reach the Champion Zone.