The Battle Arena is an optional challenge area you don’t need to beat the game. It has 17 rounds, each of which must be cleared of enemies before you can move on, and the prize at the end is the Shinning Armor. It’s harder than the final boss.
The Crucifix is the recommended sub-weapon; fill up your hearts first. You can take the Knife first, then use the effect where a duplicate sub-weapon turns into a large heart, plus heart doubling, going back and forth at the entrance to refill.
After you enter, MP quickly drops to zero, so you can barely use DSS in this area. This is the main source of the difficulty.
When you first enter the first room, your MP hasn’t hit zero yet, so you can still quickly cast one spell.
Every round is like a boss room: you can’t leave until all enemies are defeated, and you can only go forward, not back. There’s an exit after round 3 where you can give up midway, and then one every two rounds after that.
Enemies behave the same as their namesakes in other areas, but their HP, attack, defense and other stats are greatly boosted (each enemy page shows the Battle Arena stats in red).
You can’t use DSS, but you can use items to heal or recover. You can also use MP recovery items and quickly cast Summon Thunderbird before MP hits zero.
After clearing everything, when leaving left from the Shinning Armor room, stop a short distance away instead of exiting; you can turn back and challenge the rounds again in reverse, which is useful for farming cards quickly.