A unit can ambush.

A tactic can ambush I guess otherwise Band Together shouldnt exist.

http://deckbox.org/whi/Band%20Together

But can a supportcard ambush?

If you play High Elfs and get attacked and during that attack play Strike from the shadow on a support card.

Do you get to flip over that supportcard and keep it in play if you can pay the resources?

http://deckbox.org/whi/Strike%20from%20the%20Shadows

The HE tactic doesn't turn face up the development, it says that if the development is a HE unit then this unit gains ambush; that means that if the development is a legend, support, quest or tactic card, the tactic does nothing.

Thanks

If I had read the card properly, I would have seen the answer.