Unseen Strike is a very good card. It's one of the best ways for a low threat/staging area attack deck to mow through those bigger enemies. The threat/engagement cost stipulation is quite insignificant in the right decks. I have even included this card in moderate to high threat decks, because often the biggest enemies in quests have very high engagement costs.
Event. Cost: 0.
Action: Choose a character you control. Until the end of the phase, that character gets +3 while attacking an enemy with a higher engagement cost than your threat.
Even Mehl Amundsen
The Redhorn Gate #4. Tactics.