Z Because of DBZ!!
@GeneralVan: They are not omniscient or the counter-actor wouldn't have so much ease interfering with them. The very fact that they were experimenting with different universes to created a being like Tenchi and Z means that they aren't omniscient.
Was there ever a time when Time fuckery is mentioned or used in Tenchi Muyo? If not then it's a valid thing Q can do to win this fight. Honestly Q is more impressive than Z.
@GeneralVan: Theories on time travel do not equate to having a time machine or capability to tamper with the time-line. Do you have a scan of Washu having access to such a time machine? I ask this because I looked up Washu but I found no such notable inventions such as a time machine or anything having to do with time.
@CerusSerenade: No I just took that from the OBD tbh. But they do have a point. She is the smartest scientist in the universe. So something as simple as time travel probably popped up into her head first when thinking of how to kill Z. Also she could just invent real fast. And besides, even if time travel were to hypothetically work, Q would have no idea when to go back since we don't know how old Z is. This will result in him fighting Z head on, and that would end in Z slicing him down with one strike.