This can get a bit annoying. But it's doable. What I hate is what I pretty much unlocked after this.
So Clasko jumped ship. Yeah I forgot to let him back on. He decided to take over the Monster Arena and turn it into a chocobo stable...just one problem....he took over the Monster Arena. So now its up to Doormat Yuna to help whiny Clasko.
Basically you need to figure out which enemy is the real one and eliminate it. There's tons of fakes. To figure out which one is the real one just look at the monsters. Notice the direction they're all facing. The one that looks rather odd and out of place is the one that's the real one. A common mistake and forgettable one is that there's a 3rd Area in the middle as well as the room on the right when you first enter. So don't forget those. If you start having trouble the game helps you out by giving you arrows to help indicate which one is the real one.
Now that you completed this mission you unlocked a rather cool but also dangerous thing. Chocobos. To catch a chocobo its just like it was back in FF7. If you see one in battle throw some greens at it. Defeat all the other enemies and there you go. Problem arises with this. Remember before you may have previously encountered chocobo's and they just instantly run away. Well they don't do that anymore. Well not really. Instead they fight back. The worst is being hit by Choco Meteor. Which can kill you. You either need to kill the chocobo (yes you can kill them I feel no remorse about this). Or catch it. Your main goal in chocobo catching is to find about 7 or 8 Bold Chocobos that can reach level 5. I'll explain that another time when it actually matters.
Once you caught a chocobo. You can send it out to find items. When you send it out to find items go fight about 7 or 8 battles. Then come back. Your chocobo either returned or it ran away. If it came back it'll have items for you. You need about 2 chocobos to send to Bikanel Island for a specific reason and don't expect them to come back. I'll explain that another time.