A cool concept for a trap, but too expensive in my opinion. It can be compared to Hands Upon the Bow, which also lets you make a one-time attack into the staging area, but with several differences: Ambush allows multiple attackers and is not restricted to a ranged character, but also suffers from three severe drawbacks:
1) You have little control over which enemy Ambush is attached to (always a challenge with traps), whereas you can target specific enemies with Hands Upon the Bow to make sure you can actually eliminate them with that one attack (otherwise attacking into the staging area is somewhat pointless).
2) Ambush results in a Combat action, whereas Hands Upon the Bow can be played before quest resolution and can therefore even help with questing.
3) Ambush costs 2 resources, whereas Hands Upon the Bow only costs 1.
Of course there are some further subtleties to consider, but as it is I would only consider Ambush in decks with Damrod. And even at the reduced cost and with one extra card drawn it is not clear to me that Ambush would be favorable over Hands Upon the Bow. As a house rule, I would reduce its cost to 1 and/or add some defense reduction to make the card more attractive.