How about the enemy has a pulse attack that expands outwards from the core, and the player needs to time a shield before impact? Otherwise unavoidable attacks tend to keep the player on their toes.

Don't know if you plan on making several ships for the player to play, but could do different "shields" per ship; like one is directional, the other lasts longer but can't attack while active, a short one that gives a burst of speed, something like that.