The 13th level Solarion's Onslaught ability notes that you get a lesser penalty to full attack "if you have the Flashing Strikes ability."
Given that Flashing Strikes is a 7th level Solarion ability, and not an ability that would be traded out by taking an archetype, is there any reason that I'm not seeing where every 13th level Solarion wouldn't already have Flashing Strikes?