I went with the breakthrough strat, and it does look quite cool, not to mention pretty fitting for Leopold. It activates fairly late in the mission when you're supposed to be killing one of two goons to secure an escape route, so that might be why you didn't get to see it.

I cut it pretty fine with getting to Leopold and friends in the first place, though. Just about took care of Lysithea in time to get Hapi in there to heal them! (Fun fact: in case you didn't see it, Balthus has unique dialogue reacting to Lysithea if you bring him.)

Gotta say, Byleth is a hell of a lot easier to deal with when beating them is the only remaining objective and I can sic all my units on them at once!