When you first get the mega ring in X and Y, Korrina challenges you to a battle at the top of the Tower of Mastery. If you lose, do you also lose the Lucario she gives you and the Lucarionite its holding...?
As long as she doesn't get a critical hit or outspeed you, you should win by using Power-Up Punch twice, although keep in mind that even if you lose, nothing bad happens, you don't lose any money, and you'll still proceed through the storyline. After the fight, Korrina will suggest that you take the Lucario with you on your journey.
As long as she doesn't get a critical hit or outspeed you, you should win by using Power-Up Punch twice, although keep in mind that even if you lose, nothing bad happens, you don't lose any money, and you'll still proceed through the storyline.
After the fight, Korrina will suggest that you take the Lucario with you on your journey.
In other words, if you lose, you still get the Lucario and by extension, the Lucarionite
Source: http://www.marriland.com/pokemon-xy/walkthrough/plant-badge/4979-tower-of-mastery