Xerneas is the legendary mascot pokemon for Pokemon X.
You can only find Xerneas inside Team Flare’s lab. Be sure that you bring plenty of Ultra balls as well as Poison and Steel pokemon to weaken it and Fighting, Bug, and Dark pokemon to switch in once Xerneas is injured. Also be sure to save before engaging Xerneas, in case you kill it during the fight.
Xerneas is a Fairy-type pokemon so you can use Poison and Steel-type pokemon to weaken it quickly. Once you have a taken a large chunk out of it switch to attacks/pokemon that are Fighting, Bug or Dark. This way, you can safely get Xerneas into the red without worry of killing it.
Keeping track of stack status effects on Xerneas will influence your chances of capturing them. You will have to make sure the statuses don’t affect them negatively. Once you have weakened them you can start throwing Ultra balls and begin trying to capture them.
Important to not that Pokemon X legendary pokemon is that catching them is easy. It looks like you may be able to throw an Ultra ball at full health and catch them.