On the eastern side of the Calm Lands, there is an exit that will lead to a small area. There is an NPC here that talks about a Monster Arena, but all of the fiends that had been captured are no longer there. That is where you come in, as he makes a proposal to give you some goodies if you go out there and capture fiends for him, since he’s a little too old to be doing it anymore. There is a caveat, though, as in order to catch a fiend, you will need to defeat it with a special kind of weapon. Thankfully, the NPC will sell you those weapons, which come with the Capture ability on them, as well as one free slot.
The ideal setup for the weapons for capturing monsters is to buy a weapon for Tidus, Wakka and Auron. Tidus’ and Wakka’s weapons will be left alone for now, but you can spend a Lv. 2 Key Sphere to place Piercing on Auron’s weapon, as this allows you to not worry about armored fiends. Once you have your weapons equipped, you will need to capture all of the fiends at least once in the Calm Lands before the Monster Arena will unlock. Most of the fiends are common enough that you should have no trouble finding them, but a few of them are a little bit more elusive. Note that the [Mech Scouter] does not count towards the Monster Arena, as Machina cannot be captured.
The Shred is definitely one of the more elusive enemies, as this little armored bugger can really only be found near the entrance to the Calm Lands. It seems to be more commonly found by where Maechen is/was standing, on the big hill leading down to the plains. The Anacondaur can also be found near the entrance, as well as in the northwest portion of the plains area. The Ogre and the Chimera Brain can also be found in the northwest portion of the plains, although you can also find them around the building in the center.
The one enemy you might need to search for a little more is the Malboro, which seems to be more common in the northwest portion of the plains. You will, however, want to be extremely careful with them, as their Bad Breath skill inflicts all kinds of ailments on you. Getting hit by this will effectively disable your entire party and it’s a possible game over. It’s best to have a First Strike weapon on your faster character, preferably Tidus, in case of an ambush. During a normal battle, you can either use Provoke or call in an Aeon right before their turn, since Aeons are immune to the ailments.
Making Capturing Easier
Once you have found and captured all nine species on the Calm Lands, return to the Monster Arena and the NPC will commend you on a job well done. Capturing all of the fiends on the Calm Lands will unlock a special monster in the arena, the Chimerageist. It’s better to leave this until later, although it is entirely possible to defeat it now. Anyway, your reward for unlocking the Chimerageist is 60 Farplane Winds, which can be customized to a weapon to add Deathstrike. You will be putting this on Tidus’ capture weapon, meaning that he will insta-kill enemies susceptible to instant death and instant capture them at the same time.
That takes care of Tidus’ and Auron’s capture weapons, so that only leaves Wakka’s. You will be wanting Stonestrike on it, but that requires 60 Petrify Grenades. Thankfully, you have already captured an Anacondaur in the Calm Lands, so you can skip having to hunt it down among the sea of other random encounters. Instead, you can actually go to the Monster Arena and spawn an Anacondaur there. It costs 1,125 Gil to spawn one and you will receive 750 Gil for slaying it, so you’re not losing out on much money there. Instead, you’re going to be stealing from it, with the common steal being a single Petrify Grenade and the rare steal being two of them.
If you crafted an armor with Master Thief, then have that character steal, since they will always get two Petrify Grenades. You will need 60 of these Petrify Grenades, so if you started with none, then you will need 30 battles at most, but it is possible to get more than one successful for each battle. Once you finally have the required number, you can put Stonestrike on Wakka’s capture weapon. That’s about all that’s needed for opening the Monster Arena.